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Q: Why is my fish sick and how do I prevent more illness?
A: Probably 80-90% of diseases in captive fish can be prevented by
avoiding stress. Stress weakens fishes' immune systems, leading to
increased susceptibility to disease. Actually, diseases and pathogens are
almost always present in tanks, but a healthy fish's immune system will
prevent them from being a problem. Some of the most common stressors for
captive fish are:
- Poor water quality: measurable ammonia or nitrites, or very high nitrates.
- The water temperature is fluctuating more than 2 deg F/day
- Incompatible species in the tank.
- Too many fish in the tank (5 adult angelfish in 10g tank).
- The tank is too small for the fish (foot long fish in 10g tank).
- The water is too warm or too cold for the species (goldfish vs. tropicals).
- wrong pH for species (Discus vs. African cichlids)
- pH fluctuations greater than 0.2 units/day.
- Insufficient cover or hiding places present.
- Wrong water hardness for the species (Discus vs. African cichlids).
- Insufficient oxygen in the water.
- Improper fish nutrition (wrong food, foods not varied).
Q: Do I need a quarantine tank for new fish?
A: Quarantining new fish is a good habit for all aquaria, but is not
absolutely necessary for success. Quarantining is simply keeping a fish
in a separate tank for long enough to be certain that it is disease free.
Many beginners do fine without a quarantine tank, and object to the cost
of another setup. A quarantine tank does cost more, but if a hobbyist has
hundreds of dollars invested in fish, it is cheaper to have a separate
quarantine tank than to replace fish killed by a newly introduced disease.
Also, many of us become attached to fish and do not want to expose our
pets to diseases from newcomers, no matter what the cost.
The purpose of quarantining is to avoid introducing new diseases to a
stable system, and to be able to better observe new fish for signs of
disease. A quarantine tank can also double as a hospital tank for sick
fish. Hospital tanks are good because they lower the cost of using
medicines and keep diseased fish separate from healthy ones. Quarantine
is probably most important for saltwater tanks/reef systems because of the
difficulty of treating diseases, or wild-caught freshwater fish because
they are probably not disease-free. Quarantining itself can stress fish
so be sure quarantine is as stress-free as possible.
To set up a quarantine or hospital tank:
If possible, quarantine all of your new fish for about three weeks.
During that time, gradually acclimate the fish to your tank's parameters:
hardness, pH, salinity, temperature, etc., and watch for and treat any
signs of disease.
- Keep an extra filter -- a sponge filter is ideal -- or piece of filter
floss in an established tank, so that you don't have to keep the
quarantine tank set up at all times.
Some people choose instead to keep the filter going with guppies or
danios (for freshwater) or mollies (for saltwater).
- If you don't keep the tank running, use old
tank water to fill the tank. So: old tank water + established
filter = instant established tank.
- Add a spare airpump and heater. If you haven't messed with the
heater during storage, it should come to wherever you had it last
- Consider using Amquel or equivalent when medicating the tank in case the
biological filter bacteria are sensitive to the medication. Sick fish
are especially susceptible to ammonia. (Note that ammonia which has been
bound with Amquel still shows up on a nessler ammonia test. So, if you
are planning on testing for ammonia in that tank, you need to use a
salicylate ammonia test.)
- For a hospital tank, do small, frequent water changes (even every day).
Do not medicate quarantined fish ``just in case.'' Only treat evident,
definitely identified diseases. Treating all quarantined fish with a
bunch of medicines will just lead to weakened fish and antibiotic resistant
Once you are done with the quarantine, if you treated any especially
nasty diseases, it is good to disinfect the tank and reestablish the
filter. Chlorine bleach or strong saltwater (for freshwater) work well.
Be sure all traces of bleach are rinsed off. Another good disinfectant
is potassium permanganate (Jungle's Clear Water is one commercial way to
If you choose not to quarantine, do not add store water to your tank with
the new fish (see the
for acclimation ideas).
Q: How about quarantining plants?
A: Plants can carry diseases into a tank, too. It is a good idea to
disinfect new plants if there were fish in the tank with them at the store.
Refer to the
for disinfection methods.
Q: How do I avoid introducing diseases in the first place?
A: Never buy sick fish from a store. Especially do not buy fish or plants
from a tank if *any* fish in the tank shows any signs of disease or if
there is medicine in the water (water is colored yellow, green, or blue).
Store people may say the fish are fine, but if they were, why is the
medicine in the tank? Also ask how long the fish have been in the store.
New arrivals may be carrying diseases that have not shown up yet. It is
better to wait a couple of weeks before purchasing the fish. If you must
have a fish that just came in, be especially sure to quarantine it
Most important: watch your fish and know what their normal behavior and
appearance is. If you don't know what normal is, you can't know what
I suggest setting up a fish medicine cabinet. It seems like fish always
get sick when the store is closed.
- Clamped fins (fins are held abnormally close to body)
- The fish refuses its usual food for more than 2 days.
- There are visible spots, lesions, or white patches on the fish.
- The fish gasps at the surface of the water.
- The fish floats, sinks, whirls, or swims sideways.
- The fish shimmies (moves from side to side without going forward).
- A normally active fish is still.
- A normally still fish is very active.
- The fish suddenly bloats up, and it's not due to eggs or young.
- The fish is scratching against tank decorations.
And for fish big enough to handle:
- Water quality test kits: pH, ammonia, nitrite, nitrate
- Aquarium salt (NOT table salt. Most table salts contain additives to
keep them from clumping. Kosher or rock salt is OK).
- Malachite green/formalin ich remedy
- Methylene blue
- Chlorine bleach for disinfection
- Maybe one antibiotic (Kaynamycin or Furanace)
- Antibiotic-containing food
- Copper remedy for parasites
Fish are gasping at the surface, or very inactive, but there are not
visible lesions when it first starts. Their fins may be clamped. Many
fish of different species are affected, and possibly the whole tank. If
the water has been bad for a while, the fish may have finrot, or streaks
of blood in their fins.
- Malachite green or mercurochrome
Depending on your test results, try the following:
- If fish are gasping at the surface, or have purple gills: high
ammonia or low dissolved O2 may be the problem; test ammonia, dissolved O2
- If the main symptom is inactivity: test nitrites, pH, dissolved 02,
Symptoms: Fish look like they have little white salt grains on them and may
scratch against objects in the tank.
- Change enough of the water to reduce ammonia levels to 1-2 ppm
for freshwater or below 1 ppm for saltwater. If that means changing more
than a third of the water, be sure the water you add is the same
temperature, salinity, hardness and pH of the tank water. It is also
okay to do multiple smaller water changes for a few days. Aerate, and
make sure pH is at or below 7.0 for freshwater tanks. In addition to or
instead of changing water, you can also add a dose of AmQuel
to give fish immediate relief. Find out why ammonia is
present and correct the problem.
- Change enough of the water to bring nitrites down to below 2 ppm (as
with ammonia, if this is a lot of water, match water parameters or do
multiple water changes), add 1 tbsp/gallon salt (not all fish may
tolerate this much -- start out with 1 tsp), and add supplemental
aeration. Find out why the nitrite levels are high and correct the
- Change water and clean the filter. If your filter is dirty, there
is more waste material present to break down into nitrate. Start
feeding less and changing water more often.
- Low oxygen
- Run an airstone. If this helps a lot, the fish probably don't have
enough oxygen in the water. Your tank may need cleaning, fewer fish, or
additional water movement at the surface from a powerhead, airstone, or
- Improper pH
- If pH is too low: make sure carbonate buffering is adequate -- at
least 5dKH. In general, adding baking soda at 1 tsp. per 30 gal.
raises dKH about 2
degrees. For a 10-20g tank that just needs the pH a little higher, try
about a quarter teaspoonful. If that isn't enough, add up to a
teaspoonful more. You can scale this up to 1 tsp/30 gal for larger
tanks. If the pH is still too low and the KH is at least 5-6 dKH, clean
the tank. For long-term buffering in saltwater and alkaline freshwater
systems, add crushed coral. If pH is too high, pH down (phosphoric
acid) can be added. Don't rely on this stuff, except in extreme
situations like ammonia poisoning because it can cause excessive algal
growth. To lower pH long-term, filter over peat, or use distilled or
deionized water mixed with your tapwater.
White spot disease (Ichthyopthirius multifiliis) is caused by a protozoan
with a life cycle that includes a free-living stage. Ich grows on a fish
--> it falls off and attaches to gravel or tank glass --> it
reproduces to MANY parasites --> these swarmers then attach to other
fish. If the swarmers do not find a fish host, they die in about 3 days
(depending on the water temperature).
Therefore, to treat it, medicine must be added to the display tank to kill
free-living parasites. If fish are removed to quarantine, parasites
living in the tank will escape the treatment -- unless ALL fish are
removed for about a week in freshwater or three weeks in saltwater
systems. In a reef tank, where invertebrates are sensitive to ich
medications, removing the fish is the only option. Some people think that
ich is probably dormant in most tanks. It is most often triggered by
Remedy: For most fish, use a medication with formalin and malachite green.
These are the active ingredients in many ich medications at fish shops.
Some products are Kordon's Rid Ich and Aquarium Products' Quick Cure.
Just read the label and you may find others. Check for temperature
fluctuations in the tank and fix them to avoid recurrences. Note that
tetras can be a little sensitive to malachite green, so use it at half
Use these products as directed (usually a daily dose) until all of the
fish are spot-free. Then dose every three days for a total of four more
doses. This will kill any free-swimming parasites as they hatch out of
Another remedy is to raise the tank temperature to about 90 deg F and add
1 tsp/gallon salt to the water. Not all fish tolerate this.
Finally, one can treat ich with a ``transfer method.'' Fish are moved daily
into a different tank with clean, conditioned, warmed water. Parasites
that came off of the fish are left behind in the tank. After moving the
fish daily for a week, the fish (presumably cured) can be put back into
the main tank. The disadvantage of this method is that it stresses both
fish and fishkeeper.
Fishes' fins turn whitish and die back. Fin rot often follows damage or
injury. It can also be caused by poor water quality.
Remedy: First, fix the water and remove any fin-nipping fish. Change some
water (25% is good) and add 1 tsp/gallon salt to promote healing. If bad
water quality or an aggressive tankmate was the problem, that should be
adequate. Healing will begin within a couple of days.
If it worsens, decide first whether it's fungal or bacterial. Fungal
finrot looks like clumps of cotton on the fins and usually follows
injury. It is commonly seen in African cichlids or fish that have
injured themselves against decorations. Bacterial finrot is whitish, but
not cottony (unless it's columnaris), and can be contagious. The fish then
need to be removed from the tank and medicated.
Fungus: For fish large enough to handle, catch the fish, and dab malachite
green directly on the fungus with a Q-tip. This is extremely effective.
Repeat treatments may be necessary.
For small fish, a commercial fungicide such as Maroxy may work.
For severe infestations, try a bath in methylene blue (enough so you can
barely see the fish) until the fungus turns blue or for 20 min. If you
add methylene blue directly to a tank, you will kill plants and trash your
Bacterial: Antibiotic treatment in a quarantine tank. This is stressful
for the fish, and doesn't always work, so be sure of what you are doing
before you attempt it. If the fish is still eating, the best bet is an
antibiotic food. Tetra makes one that works well -- just buy the one
for bacterial diseases and follow the directions on the can.
If the fish is not eating, a bath treatment is necessary. A combination
of Kaynamycin and Furanace usually works, especially for Columnaris.
Again, treat in a separate tank and aerate heavily.
Cichlids and other ``scrappy'' fish may sustain injuries that are severe
enough to draw blood from fighting. Other fish may run into tank
decorations, walls, or rocks.
Larger fish can be netted and their injuries dabbed with mercurochrome
(available at drug stores) or Betadine (iodine-based antibiotic also
available at drug stores) to help prevent infection. Be sure to keep
these chemicals off of the gills and eyes. For really small fish, put the
affected fish in dilute methylene blue (pale blue) and 1 tsp/gallon salt
in a separate tank. If you want to keep the fish in the main tank just add
salt, as methylene blue will trash your biological filter.
Watch the fish to be sure injuries are healing cleanly, and repeat the
mercurochrome dosage if necessary. If finrot or fungus sets in, see the
above section on finrot.
Fish swells up like a balloon and may show popeyes. It may recover with
no treatment and may die despite it. The swelling is because the fish is
absorbing water faster than it can eliminate it, and it can be caused by
many different problems. High nitrates are one thing to check. Internal
bacterial infections, including fish TB, are other possibilities. If there
are no water quality problems, you may want to attempt antibiotic
treatment in a separate tank.
This disease can affect discus, other cichlids, and many saltwater fish.
The fish develops holes in it's head and sometimes along its lateral line.
Causes are unclear but as in any disease, stress and poor water quality
likely play a role. The Manual of Fish Health
states that HLLE is
probably due to nutritional deficiency, especially of vitamin C. Fish in
planted tanks rarely get HLLE, which supports the nutrition idea, since
fish can nibble on the plants and obtain extra nutrition. Untergasser also
observes that the protozoan Hexamita can be found in the lesions.
Untreated cases can eventually prove disfiguring or fatal.
Remedy: First, make sure water quality is optimal and reduce stress.
Stopping carbon filtration may help as it can remove nutrients from the
water. Then feed a vitamin-enriched food, paying particular attention to
vitamin C supplementation.
For stubborn cases, some books suggest metronidazole (Flagyl) to eliminate
Hexamita (a mildly pathogenic protozoan) from the lesions. Your mileage
may vary with that one. Metrozole and Hex-a-mit are commercial medications
Fish floats upside-down or sideways. This is particularly common in fancy
goldfish because of their bizarre body shapes. Dry food eaten quickly
swells up in the fish's intestine and keeps the fish from controlling its
swim bladder properly.
To help, feed the fish pre-soaked or gel-based foods. Green foods are
also helpful; peas in particular.
As with finrot, these disorders can also be caused by bacterial
infection. Treatment is much the same. Use antibiotic food if the fish
is eating, or add antibiotic to the water in a quarantine tank if the
fish is too sick to eat.
Add a copper remedy to the tank and monitor it with a copper test kit.
Also, Mardel's Maroxy works well. For anchor worms or leeches on pond
fish, remove them from the affected fish with tweezers and swab the area
with mercurochrome to prevent infection.
Fish look like they have been finely dusted with flecks of gold. Fins
may be clamped and the fish may shimmy.
Treat with an anti-parasitic medication such as copper or
The Manual of Fish Health
Dr. Chris Andrews, Adrian Exell and Dr. Neville Carrington.
New Jersey: Tetra Press, 1988
This is an outstanding book, and I highly recommend it to anyone who is
interested in reading about fish disease.
Handbook of Fish Diseases
Translation by Howard H. Hirschhorn
T.F.H. Publications, Inc., 1989
This is my second-choice disease book. It is very good, but some of the
treatments may be difficult to obtain, and it goes into more detail than
the average hobbyist needs (or wants) to know.