The kinds, numbers, sex and size of the pet fish you select for your aquarium or pond are among the most important decisions you'll make as a fish owner.

Having an idea of your expectations and resources, combined with the advice of a knowledgeable and helpful pet store clerk, will go a long way in making pet fish selection easy, productive and rewarding.

Freshwater or Marine Fish?

Most beginners and, in fact, most pet fish hobbyists, maintain freshwater fish in aquariums. The marine fish portion of the hobby represents less than 10 percent of the total pet fish industry. So if you're just starting out, you'll probably want to stick with freshwater fish for a while for simplicity's sake. As a rule, marine fish are more expensive, less hardy, and, of course, require artificial seawater, which can be expensive.

The Community Aquarium

One of the more popular approaches to pet-fish keeping is the "community aquarium." As the name implies, this is an aquarium, usually of moderate size (20 to 50 gallons or more), that houses between 3 and 30 fish that are generally non-aggressive. Most fish in a community tank will eat flake or pelleted food and will thrive in water with "middle-of-the-road" pH, hardness and temperature values. See the list at the end of this article for some sample community aquarium species.

The Aggressive Aquarium

This type of aquarium usually houses a small number of larger fish. The fish that live in this type of tank may attack or even consume smaller or more submissive fish. Many "aggressive" species are quite attractive, live a long time and have unique and interesting personalities. When mixing aggressive species in an aquarium, all fish should be of comparable size, and there should be some structure (plants or driftwood) so these fish can "stake out" territory and hide, if necessary. Your pet store dealer can help you make decisions on which fish from this category to place together.

The Single-Pet Aquarium

Perhaps you want an aquarium but are only interested in a single pet fish? This is one of the easiest and most rewarding ways to be involved with the pet-fish hobby. For the most part, fish aren't social creatures and don't require contact with other fish, except to reproduce. In other words, fish don't get lonely when there aren't other fish around. In fact, your single fish might be much happier not having to compete with other fish for food, space and your attention.

Some Good Community-Aquarium Varieties

Mollies

Guppies

Swordtails

Gouramis

Silver dollars

Tetras

Corydoras catfish

Small plecostomus species

Danios

Loaches

Koi (primarily for the pond)

Goldfish -- there are numerous varieties available
        
Some Aggressive Aquarium Species

Oscars

Jack Dempseys

Red devil cichlids

Jewel cichlids

Convicts

Large plecostomus species

Green terrors

Piranhas (illegal in some states)

Carnivorous catfish

Single Pet Aquarium Species

Goldfish

Any of the aggressive species

Pacu

Large catfish