If we define a good beginner's fish as one that is easy to feed and care for, hardy, able to live in a variety of water conditions, and attractive, then there are a number of widely available fish which fit the bill nicely. Many of these are regularly sold as beginner's fish. But watch out! Many of the fish sold as beginner's fish really are not well suited to that role.
Many of the smaller schooling fish make ideal first fish. These include White Cloud Mountain Minnows, the several commonly available species of Danios and Rasboras, and most available species of Barbs. For those with a slightly larger tank, Rainbowfish make a great schooling fish. Corydoras Catfish are ever popular schooling catfish.
While many beginners are tempted to get just one or two of each of several different schooling fish, this should be resisted. Schooling fish do better if there are several of their own species present for them to interact with. A minimum of six of each of the midwater schooling fish is recommended, while four is the bare minimum for Corys. In the long run, a school of a dozen fish showing their natural behavior will be more pleasing than a mixed group of fishes unhappily forced to share the same tank. (``Mom, why is that one fish hiding behind the heater and that other one just hanging in the corner?'')
``Slim-bodied'' could be fish such as neon tetras, White Cloud Mountain Minnows, danios etc.; ``medium bodied'' might be red-tailed black sharks, tiger barbs, platys, cory cats etc.; ``heavy bodied'' would be goldfish, oscars etc.
In other words, this is only a rule of thumb, and the ``maximum'' population that is safe and humane will vary from tank to tank. Factors that increase your possible fish load include:
The bio-filter for your tank is only ``fed'' by the wastes of the fish you have in the tank. This means that no matter how large your filter (e.g., one rated for a fully stocked 50 gal tank on your 20 gal), the bacteria population will be limited by the ``food'' it has. Few fish = small bacteria population.
We are accustomed to thinking of bacteria reproduction as ``explosive''. Many bacteria can double their population size in hours, after all... but as we have seen in the CYCLING SECTION, the appropriate nitrifying bacteria are relatively slow to reproduce. There will be a time delay between the increased waste production of additional fish, and increased waste processing by the bacteria. In extreme cases, the ammonia increase could harm or kill your fish before the bacteria population had time to ``catch up'' to the amount of available nitrogenous wastes.
This is why it is wise to add fish slowly and gradually. Safely bringing your tank's population up to the maximum load can take more than 6 months; in fact, it should be permitted to take at least that long. Leave breaking the rules to those with more years experience than they have fish.
After you have the fish acclimated to your tank's water chemistry, there are a couple of things you can do. You can place the fish directly into the main tank and hope for the best, or you could place the fish into a quarantine tank. In either case, quickly net the fish from the bag to the tank so that no store water gets transferred to the tank.
The best scenario is to place the fish in quarantine. Keep the fish in the quarantine tank for 2 weeks and watch for signs of disease. If the fish gets sick, you can medicate the quarantine tank without affecting the chemistry of the main tank's. If you are going to quarantine the fish, you should acclimate the fish to the quarantine tank's chemistry, not the main tank.
While a quarantine tank is a good idea, it is most likely that you do not have such a luxury (for now, at least... :). In this case, be extra careful to select healthy fish at the store, and carefully monitor your new arrivals for the first few weeks in your tank for signs of stress and disease. You always risk infecting the other fish in your tank when skipping quarantine.
Fish food is somewhat delicate. Exposing it to sunlight, leaving the lid off so that damp can come in, or buying a very large container that takes 8 months to use up all can sabotage the nutritional value of your fish's food. Generally speaking, there are five classes of fish food:
Take it seriously. The reason most folks have fish is, we hope, to observe them. If not up close and personal, at least in a general sense. The perfect time to do some of your observing is when you feed. Each time you feed, park yourself in front of the tank to watch. Put in less than you think can possibly be enough. Watch the fish consume it. Observe what falls to the bottom. If you don't have any fish who are primarily bottom feeders (pl*cos, corydoras, loaches etc.), take the time to learn if any of your other fish will glean the bottom; gouramis often will, but rainbows generally won't, for example. If you do have bottom feeders, watch to see how fast they eat.
So you put a little pinch in, and after 2 minutes (you counted!) there is practically no food to be seen... except a little on the bottom which the cories are really going for. Yep, you can probably safely give them some more. But watch to make sure they really eat all of the second pinch too. It is better to feed a tiny bit several times a day, especially with fish who won't scour the bottom, than it is to feed a bunch all at once... but most adult fish will do fine being fed a 5 minute ration once a day. In an established tank, even less often is preferred by some fish keepers; that way, the fish will eat more of the algae and other edibles that can naturally occur in a tank.
Another thing to keep in mind: fish CAN get fat, especially if fed a lot of rich foods such as bloodworms. Many of the fish you'll buy to put in your tank are juveniles: how they develop into adult fish will be determined by your care of them. Just as high nitrates can stunt a fish's growth, shorten its life, and prevent it from ever breeding successfuly, fish who are overfed can end up with deformed bodies and other problems - plus they poop more... which has obvious ramifications :-). Feeding a good variety of foods ensures that your fish will get not only the rich foods, but also fiber (brine shrimp and other crustaceans) and vegetables (algae foods, vegetables).
A word on live foods: certain commercially available live foods are considered risky by many hobbyists, as they can carry parasites - tubifex worms in particular. You will have to decide yourself how you feel about this risk. Be very sure that you are feeding food that is, indeed, still alive! Rinse the critters thoroughly, and especially if they are not able to live in your tank water, be just as careful about overfeeding live food as you are other foods. Live foods are covered in detail (including culturing instructions) in the LIVE FOOD FAQ.
Partial Water Changes