Filters and Filtration



The FAQ owes its existence to the contributors of the net, and as such it belongs to the readers of rec.aquaria and alt.aquaria. Copies can be made freely, as long as it is distributed at no charge, and the disclaimers and the copyright notice are included.

Filtration Methods

Filtration is very important to the welfare of your guests, and as such there are many different kinds of filters. Since there are many brands and opinions, one should post requests for recommendations. Equipment that is typically found on advanced aquarists' systems are also noted. These are typically more expensive, and are generally not needed by, nor recommended for beginners.

In general, if one needs to rinse/wash filter media, one should never use soap or hot water (or for that matter, very cold water). This would serve to kill the beneficial bacterial colonies in the filter media. (See nitrogen cycle). Use the reject water siphoned out your aquarium to wash/rinse filter media.

The following filter discussions are arranged alphabetically, with related topics grouped arranged separately at the end.

These are specific examples of internal filters. These use streams of air bubbles to push water up and out of a tube, thereby creating a pull of water into the filter through the filter media. (See INTERNAL FILTERS).
These are large plastic "canisters" typically located outside of and under the aquarium in the cabinet. They are designed with a powerful pump which draws water through an in-take hose located in the aquarium, pushes it at a relatively high pressure through the filter medium, and forces it back to the aquarium through the return hose. (Many times, this water is sprayed across the surface of the water to create aeration). See the CANISTER FILTER section for more details.
Ion exchange resins are used to deionize new water. Also, synthetic resins designed to adsorb specific ions (like phosphates, or nitrates, or sulphates) are now available. See ads in FAMA and other such magazines for new items. Thiel has chapters on chemical filtration in his books, so does Moe (see Books). Carbon is also a form of chemical filtration.
These can be described as purely mechanical filters. It relies on the diatomaceous earth (skeletons of tiny animals called diatoms [Diatom comes from the Greek: diatomos meaning "cut in two"]) which "cling" onto dirt and a fine mesh screen to capture the "dirty" diatoms. Because of the efficiency, diatom filters are also called water polishers. These are not used over long periods (they clog up very quickly), and hence cannot be considered good biological filters.

Diatom skeletons are composed of silica and there has been some speculation as to whether usage in marine aquaria will lead to "brown algae" explosions.

Oftentimes, this is used to denote power filters. But this not always the case. (See POWER FILTERS)
These filters are plastic cartridges that fit inside the aquarium. Dirty water is drawn into the filter through slits located on either the top or sides of the body. The method of water propulsion can be motorized, or bubble driven. Maintenance of these filters can be difficult. Many times, as one lifts the filter out of the aquarium, dirty water backwashes out into the tank.

Some designs, like Lee's Triple Flow and similar models by Penn Plax and Marineland do not have this problem. They also cost more, of course :-) Also, one has to immerse entire appendages into the tank to access the filter. This type of filter is only recommended for up to 20 gallons. You may be surprised how inexpensive external power filters (EPF) can be and how quiet they are. Always consider power filters unless you are raising fry, spawning live-bearers or killies in a "natural" setup, or need minimal flow (for spawning bubble-nest builders).

Almost all filters perform mechanical filtration, and most perform biological filtration. The difference between these is that mechanical filters will capture and remove suspended particles from the water, clearing the water in the process. (The filter media obviously becomes dirty, and must be cleaned every once in a while). Biological filters provide habitats for beneficial bacteria that breakdown nitrogen compounds from biological waste into progressively less toxic forms (ammonia -> ammonium -> nitrites -> nitrates -> nitrogen). (This is a gross over simplification. See nitrogen cycle in H20 quality).
This is the common name for filters that hang on the back of the aquarium. Two basic types exist. In one case, water is drawn through siphon action into the filter media, and expelled by a pump. In the second case, water is drawn by a pump into the filter media and the allowed to trickle back into the tank. A major benefit of such a filter is the ease of maintenance. The location and design of the filter allows for quick and non-messy removal of dirty filter media. See the POWER FILTER section for more details.
PRE-FILTERS (advanced)
This are filters used primarily to remove most of the particles from the water before it arrives at a primary filter, which may be located in a difficult to clean area. The filter media on pre-filters are removed and replaced easily, thus reducing the number of times one has to change/maintain the media in the primary filter.
Protein skimmers, also known as foam fractioners, are an unusual type of chemical filter. They are primarily used in marine aquaria, because they require the formation of foam which form more readily in salt water. Protein skimmers can be used in freshwater, but will be less efficient, and may require greater flows of air to produce sufficient foam. They remove from the water a wide variety of organic compounds (e.g.. proteins) that normally collect on the surface of the tank water.

These compounds, surface active molecules, are attracted to the air-water boundary by their molecular structure. Large amounts of these compounds are often visible to the aquarist as slicks, sheens, or thin scum on the tank surface. In addition to the presence of films on the surface of the water, an abundance of these compounds is signaled by increased stability of bubbles floating on the water. This stability can result in large rafts of bubbles spanning the surface of the tank, and very large bubbles grown from many successive merges of smaller bubbles.

A protein skimmer consists of a space where fine air bubbles are encouraged to mix with circulating tank water, and a means for collecting the resulting foam and removing this foam from the tank. Typically, a mist of fine bubbles is allowed to rise in a column of circulating water, forming foam in a small chamber at the top of the column. This foam rises upwards through a narrow opening, and flows into a collection cup which must be periodically emptied. In a properly adjusted protein skimmer, some amount of foam is present all the time in the chamber, but will only rise high enough to reach the cup and be collected when surface-active compounds are present in sufficient quantity to stabilize the foam.

REEF FILTERS (advanced)
REVERSE UGF (advanced)
This uses the same gravel plate as a regular UGF except that water is pumped down an uplift tube (in this case a misnomer), under the plate, and then *UP* through the gravel. The water traveling through the uplift tube must be clean, since it would be very difficult to clean under the gravel plate. The water can come from the output of a filter (canister filter for example), or can come from a new generation of powerheads (submersible centrifugal water pumps) has a "reverse flow" feature which makes it easier to use them for reverse flow UGF. (See POWERHEADS). The powerhead should be outfitted with some sort of pre-filter such as a sponge.

Since the mechanical filtration will have been taken care of already, the reverse UGF is primarily a biological filter. It has the alleged benefit of keeping fish filth suspended in the water, or at least not being sucked down under the UGF plate; allowing for easier removal during tank maintenance/vacuuming.

Because most aquarium equipment was not designed to work in this manner, one typically has to rig up one's own hose adaptors to connect the output hose to the uplift tube.

Recently there have been several products designed specifically for RUGF use. Marineland is one company that sells RUGF units consisting of powerheads, prefilters and connection tubing. Other companies sell reverse flow powerheads, but it is VERY important to use only systems which have adequate pre-filtration, and not just pump water under the plates.

RO FILTERS (advanced)
These purely mechanical filters that are used to purify water from the tap before introduction into the aquarium. These devices use a thin membrane that only allows water molecules (and a few other smaller dissolved ions) to pass through to the output. The unwanted water and materials are redirected to the drain. This filtration method wastes water. RO Filters can also be found at building supply stores since household drinking water filtration also uses the same technique.

There are two common types of RO filters sold. One uses a cellulose tri-acetate (CTA) membrane, and the other uses a thin film composite (TFC) membrane. The TFC membrane is the best for aquarium use.

Because the rate of RO water production depends on water pressure, temperature, and solute concentrations it is important to realize that unless you have optimal levels of all those above you will probably only get 50-80% of the production rate advertised. Drops in temperature below the 70 degree range usually measured (winter time, for example) will drastically reduce production.

SKIMMER (see Protein Skimmer) (advanced)
This term can refer to surface skimmers as well as protein skimmers. Surface skimmers are essentially boxes set just below the surface level of the aquarium. Water "spills" into this box and is drawn out to a filter (by siphon or mechanical means). This oftentimes removes the surface "slick" found on marine aquarium.
These are open cell sponges that are connected to some mechanical device (bubbler, power head, pump etc.) that will draw water into the sponge. Because of the large surface area/water flow ratio, the suction is not strong enough at any given point on the sponge to trap fry or other small fish (as opposed to, for example, the intake hose of a canister filter). These are often used for breeding tanks etc. Fish filth is trapped in the sponge, which is rinsed every once in a while for maintenance. Most sponges are shaped in such a way that, as filth clog up particular areas, the suction collect waste in other areas. The sponge also serves as a biological filter.
The number of times the volume of water in the tank should pass through the filtration system in an hour. The general rule is 3-5 tanks/hour, unless the fish load is very light, in which case .5-1 tanks/hour is OK. Canister filter instructions suggest 1-2 tanks/hour. High turnover rates are probably beneficial (more filtration), as long as the fish are not swimming for their lives constantly.
This filter is a plastic plate with holes/slits that is placed first in the aquarium, after which gravel is poured on top. Water is drawn through the gravel, and impelled up and out through uplift tubes (typically located in the rear corners of the tank). The impelling method could be bubblers or power heads. The gravel becomes the filter media for both mechanical and biological filtration. This means that the gravel should be "vacuumed" every once in a while. As the gravel becomes clogged, water bypasses the clogged areas, and these become sites for unhealthy bacterial activity.

There has been much debate over the use of UGFs.

WET/DRY FILTERS (advanced)
These filters enhance the metabolic activity of the beneficial bacteria by providing good conditions for their growth. These are very much in favor with marine aquarists, especially reef aquarists whose guests require clean stable water.

The principle of operation is as follows: Water is sprayed (by a revolving spray bar) or trickled/dripped (by a plate with lots of holes) over a media that is largely exposed to air (i.e. not immersed in water). As the relatively random patterns of water trickle over the media, the large surface of the media is constantly kept wet with a thin film of water. The large surface area can dissolve lots of oxygen and may de-gas (expel) a number of other things, most notably ammonia (NH3) and CO2. The media houses lots of bacteria which takes the oxygen and converts NH3/NH4+ into NO2- (nitrite), and NO2- into NO3- (nitrate). This is the "dry" phase.

The "wet" section is a submersed media which can be one or both of the following: a purely biological filter, or a denitrification filter. A number of sources have been suggesting that it contributes more to oxygen depletion than to any meaningful nitrification. A denitrification filter is created by placing a large submerged sponge in the path of the water, allowing some water to *slowly* flow through the sponge, while most of the water passes over it. In the nearly anaerobic cells of the sponge, nitrites are converted to nitrogen by Pseudomonas and other bacteria (however, Thiel and others have been very critical of this method of denitrification); chemical "augmentation" -- a dolomite or other calciferous material is used to leach calcium carbonate into the water to replenish what gets metabolized by plants; and supplemental filtration, like a bag of activated carbon or some Chemi-Pure.

The water which is relatively depleted of oxygen, is passed into the wet phase, which supposedly converts the NO3- into nitrogen and some oxygen. This water then gets pumped back into the tank.

Wet/dry filters are not cheap, and because of this, the net contributors often come up with suggestions for rolling your own. Keep your eyes peeled for these messages.

Filtration Related issues

Some aquarists take the output of air pumps and bubble the air through water in a closed jar. The air is then taken through the lid into the aquarium bubbler. This serves to dissolve air borne chemicals (such as mineral oils from the pump) in the jar water before it gets into the aquarium. Also useful for households that have smokers.
These are small pellets of carbon manufactured from organic material (such as bones). These pellets contain microscopic caverns that are the right size to trap certain molecules or ions (called adsorption as opposed to absorption). After a period of 3- 6 months or so, the carbon becomes ineffective. It cannot be "re- activated" by heating in your home oven unless you have a blast furnace at home.
Ozone is a highly reactive form of oxygen, and it is bubbled through water in a special reactor to oxidize organic waste material and some toxins (rendering them less harmful). Since it is so reactive, it is invariably bubbled through carbon (so leftover ozone forms carbon dioxide) before being released into the atmosphere.
This is an organic material made of composted forest fibers. Peat is rich in humic acid and tannin, and is sort of like tea. Its effect on water similar to tea. It softens water and leaves the water slightly acidic (and yellow). It is used for lowering and buffering water for sensitive fish. Peat also helps chelate some metals and make them usable by plants. There are two varieties of peat available: "normal" peat which consists of small particles (much like peat used in gardening) and *fibrous* peat that looks like a bunch of long strands and twigs (this is sometimes referred to as "German" peat and it is nearly impossible to buy in this country except in tiny, extremely overpriced packages of Fluval and Eheim brand in aquarium shops.

Garden peat can be bought dirt cheap in bulk from garden supply stores. MAKE SURE IT HAS NO FERTILIZERS OR FUNGICIDES -- these will kill your fish. According to Oleg Kiselev, Canadian peat, especially "Sunshine" brand, seems to be safe. Oleg has also used "Black Magic" gardening peat with equal success. ALWAYS RINSE PEAT BEFORE USING and many books recommend you boil it a few times, too. Oleg usually boils peat 3-4 times and stores it wet.

Peat is very important for killifish spawning.

These are miniature pumps that draw water through uplift tubes (associated with UGFs) and impel it into the aquarium. They often come with a feature that allows air to be drawn into the outflow resulting in a fine mist of bubbles. (This is due to something called Venturi action). The water output can usually be directed in any direction, and up or down. Some even have a reverse feature for Reverse UGF systems.

Although sometimes used as pumps, these are not really designed to push water up any distance. They are designed to draw water and push it out laterally.

Ultraviolet light sterilizers are used in series with filters to kill water borne parasites (such as ich) and/or bacteria. Although not strictly a filter, it does ultimately remove harmful organisms. When used in reef tanks or breeder tanks where the occupants depend on microscopic organisms in the water for food, these should not be turned on during feeding time.
Zeolite is a naturally occurring mineral that can exchange "hard" metallic ions (like magnesium and calcium) for soft metallic ions (like sodium). This softens water. Zeolite also adsorbs ammonia. Zeolite will not work in salt water. It is reactivated by immersing it in a strong salt solution for 24 hours.


The question: "Which canister filter should I buy?" is one of the most frequently asked FAQs on *.aquaria and each time it comes up there is discussion of the relative merits of each of the major brands.

General Comments:

The Cadillac of canister filters. Very solid construction and operate virtually silently. Significantly more expensive than other brands (see below). Eheim recommends smaller models for larger tanks than the others and almost everyone buys larger than they say (2217 for a 55 gallon tank, for example, rather than the 2213 which they recommend). The Eheim canister is a single hollow tube with water entering from a 90 degree tube at the bottom and flowing upwards through whatever you've packed in and pumped out the top. They do not come with shut-off valves (a must-have for easy canister maintenance), but these may be purchased separately (approximately $30). Eheims have been dark translucent green with green hoses and black motor encasing. The hose-clamping system is very solid. Other options such as pre-filters, surface extractors and oxygen diffusers are available (and many will work with other brands). They can also be ordered with built-in heaters (freshwater use only). Made in Germany.
Fluval filters have a slightly different design than the Eheims. Fluvals have the water enter and exit from the top of the filter. The entering water flows down around a central core which contains the filter media, and then returns up through that core to pass through the media. The modular media containers which fit into the Fluval filter allow for easy changing/cleaning of any segment of the media without disturbing the others. However there may be some small degree of media bypass from this design. Flow rates are higher than the Eheim models. Fluvals come with included on/off valves and a diffuser bar. They recently were changed to a dark grey/black translucent color with beige hoses. Slightly older models are brown/tan with orange hoses. There is no difference between the models except the color. Fluvals are also very quiet, though not absolutely silent like the Eheims. I sleep with one 2 feet from the head of my bed and can't hear it at night, however. The hose-attachment clamps are screw-down type and can be hard to turn. They are secure if attached correctly. Made in Italy.
Magnum recently changed their design from the 200/330 series up to the more modern 220/350. The older models required oiling and generated a fair amount of noise compared with the others. The new models are magnetic impeller driven just like the Fluval and Eheims. There is also a new Magnum product called the "HOT" magnum (Hang-On-Tank) which I have not evaluated. It appears to be an over-the-back filter which has a lower flow rate (250 gals) than the bigger Magnums, but require no hoses and may be easier to set up. Magnums have traditionally been the cheapest option when buying a canister filter. Their design has the water entering the top of the filter, and being forced through a central container from the sides (as compared with bottom to top in the Fluvals) then to be pumped out the top again. Magnum filters utilize a "cartridge" system which includes carbon containers and micron cartridges. The micron cartridges can be used with diatom powder to provide diatom filtration (see Diatom filters). The filter cartridges are usually covered by a floss sleeve to provide coarse mechanical filtration. Magnum filters provide a high flow rate but hold substantially less filtration media than the others. Construction is generally considered to be of cheaper materials than the other filters, with hoses being held on with rubber straps rather than screw down valves for example. Magnums usually are a hot debating point in the canister filter selection process: some people have had wonderful experiences with them and others have not been so lucky. Made in America.
Some functional information is below. Numbers taken from manufacturers literature. Keep in mind that flow rates will vary depending on what the unit is packed with and how high the water must be pumped. The numbers listed below should be considered the maximum possible (they are generally listed with no media in place. Additional back-pressure will reduce flow substantially, especially on smaller models).

        GPH   Electric   Manufacturers  Cost (FAMA)
               Usage     Recommended    (US$ 1/94)    Filtering
              (watts)    Tank Size                    Volume
2211:   80       5w      <40 gal.        $51            1 L
2213:   116      8w      <65 gal.        $56            3 L
2215:   165      15w     <90 gal.        $90            4 L
2217:   260      20w     <125 gal.       $130           6 L
2250:   380      ??      <265 gal.       $200           ??
2260:   500      ??      <400 gal.       $260           ??
103:    103      7w      <25 gal.        $46            1.18 L
203:    111      7w      <50 gal.        $44            2.22 L
303:    222      15w     <70 gal.        $58            3.70 L
403:    317      22w     <100 gal.       $88           6.48 L
HOT:    250      ??      ??              $58            ??
220     220      ??      ??              $45            ??
350     350      ??      ??              $55            ??


A power filter is a box shaped filter that is hung on the back of an aquarium. An intake pipe projects down into the tank and the output of the filter flows from spillway(s) on the body of the filter over the side of the tank and into the tank. The other major type of filter that is similar to a power filter is the canister filter.

Why use a power filter?

  1. Space: These filters can be small relative to the volume that they pump through the filter. They hang neatly over the side of the tank. Most people put them on the back, where no one can see them, and where the space is wasted anyway. *WARNING* All filters claim to pump a certain volume per unit of time. This figure is sometimes tested by using clean (or no) filter media. The actual volume pumped will start at the published value and decrease to zero if the filter media is not cleaned or changed.
  2. Noise: Power filters seem to be relatively quiet compared to other popular filters.
  3. Cost: The initial cost for a power filter is low.
  4. Easy of maintenance: Most of the filter media for a power filter is available prepackaged (but at additional cost). Power filters provide easy access to the filter and media because it's part of the tank, in a convenient location. Disassembly of the filter is straightforward and simple. Some filters will also allow you to insert media that was not specifically designed for that filter. Some filters are designed for you to throw away your biologically active media if you follow the manufacturer's instructions (this is bad, see below).

Major Manufacturers of Power Filters

	(GPH = US gallons/per hour).
	Prices are approximate mail order prices in $US.

		Name 	GPH	$	GPH/$
Aquaclear	Mini	100	10	10
(Hagen)		150	150	14	10.7
		200	200	14	14.3
		300	300	21	14.3
		500	428	38	11.3

Whisper		Compact	100	10	10
(Second Nature)	1	150	13	11.5
		2	200	14	14.3
		3	300	21	14.3
		5	400	31	12.9

Penguin		110	110	12	9.2
w/biowheel	160	160	14	11.4
(Marineland)	300	300	22	13.6

Canistar	MX	155	18	8.6

Supreme Aquamaster PME 	120     14	8.6  	
        Aquamaster PMSW  ?      25  	
        Aquamaster PLSW 300     29  	10.3	
        Aquaking        300     32  	9.4
        Superking       600     41	14.6

Mechanical Filtration/Chemical Filtration

The key to mechanical and chemical filtration with a power filter is the amount of media that the water must pass through. Ideally, all the water that passes through the filter also must pass through all of the media.

The Aquaclear and Canistar filters should work the best at mechanical/chemical filtration. They push all the input water through all of the media. The Aquaclear will allow water around the media if the media is clogged. Look for back-flow coming out near the siphon tube. The Whispers and Penguins (except the Penguin 300) design allow some of the water to pass by the chemical filtration (carbon, ammo-chips etc.), but all the water must pass through the dacron mesh. The Penguin 300 has a "media basket", 2 removable containers in the filter body so you can add your own media to help filter the water, this design addition makes it difficult for any water to pass by the media. The diatom water polishing mode on the Canistar seems to be iffy.

Biological Filtration

The main concern here is whether or not you throw away the bacteria's happy home when it comes time to clean the filter. There is probably little difference in the performance of dacron vs. foam for harboring bacteria.

The Aquaclear has a foam insert to harbor helpful bacteria. You may rinse it and put it back in the filter, without ever buying new foam. No helpful bacteria are lost if this is done correctly. Also the "force all the water through the filter" concept discussed above applies here too, all the water must pass over the bacteria, which may clean it better. A caveat is that the fast flow through the sponge does lower the ability of the filter to get rid of NH4 in 1 pass, but the increased volume over a typical canister filter may make up for that. The Whisper's, Penguin's and Canistar's' dacron pad eventually must be thrown out, thus losing the bacteria. Ingenious solutions have been invented to minimize this problem, but IMHO the Aquaclear is superior in this regard. The Penguin is like the Whisper, except that it has an added device called a Bio-wheel. It probably helps, but there are mixed reports on this. The wheel may need regular, infrequent maintenance to prevent clogging.

The Supreme models are unique in that they do not use the troublesome magnetic-impeller water-immersed motors. Their motors are large industrial air-cooled ones that sit on top of the filter and run the impeller via a plastic shaft. Also, they use siphon tubes to bring the water into the filter box, and pump the filtered water back to the aquarium, preventing the impeller from shredding and thus reducing the particle size in the influent.

Other gotchas/Special features

The Aquaclear has good sponge media, and allows the use of media bags so you can fill your own. The carbon bags cost a bit, but they seem to use good carbon.

The Penguin biowheel has a tendency to stop if it gets dirty. If the wheel isn't' turning, it isn't working.

Power consumption

Not all equipment really consumes the amount of power that it's rated for. If you want an accurate measurement of your power consumption, it may be possible to borrow a Kilowatt-hour meter from your local utility company. Not everyone answering the phone will know what you want, but be determined.

The meter is placed in line with your equipment, so you will need to rewire your tank so that all of the items to be monitored are plugged into the meter. The meter is similar to the one that comes into your house, except that it is configured for 120V 2 wire. The meter has a granularity of one kilowatt hour, so if you're measuring something that draws little power, it may take a while to get an accurate reading.


There are no clearly superior filters here, although the Whisper seems to lose in almost every category (your mileage may vary). The Aquaclear series is probably the best overall, however, if you need a big filter, the Penguin 300 may be better because it has the Bio-Wheel and the media baskets.


Keep away from cheap pumps. Make sure they are UL listed (as you should with all other electrical devices you are planning to use with your aquarium). Listen to the pump (under load!) before you buy it. If you can hear it in the noisy petshop, imagine what it will sound like in your silent house at 2 AM. It is also a good idea to get a pump that is not too much more powerful than you need - more powerful pumps are invariably more noisy.

As a general rule, "adjustable" air pumps are worth extra money only if the adjustment is electronic, rather than by a by-pass valve. The former is more quiet. Make sure your pump is positioned above the water level of your tank, because power failures and other events can cause water to back-siphon into the pump and flood it. You can also use "check-valves" available at pet shops to make certain that this does not happen.

The following is from Spass Stoiantschewsky:

and the last name is spelled "stoiantschewsky", my system
has it spelled wrong...always has, probably always will...

the address will probably change, but i haven't the faintest
idea should have already changed...

best pump tested at depth to date: Tetra luft g 
best pump volume (shallow depth) to date: whisper 1000
noisiest pumps: whisper
quietest pumps: challenger (?), followed closely by Tetra and

best buy in pumps: Tetra luft g

noise level measurements are purely by ear.

i'd like to talk to people who have pumps they particularly like 
or dislike and anyone who has pumps that haven't been tested (easy
untested pumps: schego, iwaki, wisa, supra 
tested pumps: whisper, silencer, Tetra, challenger

[Ed. Note.  Spass will post a more complete article soon. ;-) ]


Water Temperature Pull Down From Room Temperature (Degrees Fahrenheit) *
               5o      10o      15o      20o      25o
             ----     ----     ----     ----     ----
1/6           200      100       67       50       40
1/5           560      280      187      140      112
1/4           800      400      267      200      160
1/3          1000      500      333      250      200
1/2          1440      720      480      360      288
3/4          2600     1300      867      650      520

		              Tank Size     
* Tank sizes represent the maximum volume for each horsepower and
temperature combination.  To provide a margin of safety, add
20 percent to the tank volume when determining chiller requirement.

Reprinted with permission from _Aquarium Fish Magazine_, Vol. 5,
No. 3, December 1992.