How to Choose Aquarium Fish

choosing fish, compatibility
The compatibility of the different kinds of fish in your aquarium is vital to their survival. See more aquarium pictures.
©2007 Publications International, Ltd.

A visit to any large aquarium or pet store will reveal an astonishing variety of fish -- a couple hundred species or more. Of these many possibilities, only some are suitable for new aquarists.

Never purchase fish without knowing enough about them to determine how well they are likely to do in your particular aquarium. Often, hobbyists find themselves leaving a pet store with a fish that they just couldn't pass up without having any idea what it eats, how large it gets, or if it has any unusual care requirements. While it is always fun to acquire a new fish, doing it haphazardly can be disastrous and costly.


When choosing fish, there are a number of factors to consider. For example, fish that are found only in very specific habitat conditions are not going to do as well in an aquarium that differs much from their natural environment. Species that have adapted to a wide variety of conditions in nature tend to be the hardiest fish in an aquarium.

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Your fish also need to be compatible with each other. Just about every new aquarist starts out by keeping a community aquarium containing a variety of fish that differ in size, shape, and color. Often, the fish in such a tank originate from different parts of the world and have adapted to different water conditions. Their behavior patterns and food preferences may also differ.

This means that even if all of the fish are hardy, if they are not compatible, there will be difficulties. Compatibility is essential to success within a community setting.

Compatibility is generally assumed to mean that the fish get along. In other words, the fish are peaceful and do not harass one another. Fish that are subjected to harassment on a continual basis are under so much physical stress that they are far more likely to become sick, no matter how good the water quality is.

As important as this basic compatibility is, however, your fish need to be compatible in other ways, too.

If the fish prefer very different water conditions -- some doing best in hard, alkaline water and others doing better in soft, acid water -- you will not be able to provide the best environment for all of them. The fish least compatible with your aquarium water chemistry may not grow as large or exhibit their best color.

Behavior patterns are also important. Fast-swimming fish will disturb and upset fish that swim slowly. Fish that are aggressive swimmers will often take more than their fair share of food at feeding time. The more timid species will hold back and thus get little to eat.

Adding more food to compensate only increases the problem of uneaten food in the tank, making it more difficult to maintain good water quality.

Behavior differences can be very disruptive. Even when fish have peaceful dispositions, conflicts can still arise among them in the tank. Fish that are territorial will keep other species from entering their area, limiting the amount of space for all the others.

If there aren't enough hiding places, more dominant fish will maintain control of the few that do exist in the tank.

Get more helpful hints on choosing aquarium fish on the next page.

To learn more about freshwater aquariums, see:


Choosing Aquarium Fish: Helpful Hints

choosing fish, hints
The choices for new fish are limitless, but take care before introducing a stranger into your aquarium community.
©2007 Publications International, Ltd.

Fish that have different food requirements can make it difficult to supply each species with the appropriate diet. Those that require lots of protein will need different foods than those that require more vegetable matter. Growth and health are tied directly to a nutritionally balanced diet, and the proper balance can vary from one species of fish to another.

Keeping fish of vastly different sizes in the same tank, no matter how peaceful, is an invitation to disaster. Fish will eat whatever fits in their mouth.


Many a novice has been shocked to discover that the angelfish and tetras that coexisted so well when they were small are no longer safe together -- the increasingly larger angels have no qualms about consuming the smaller tetras.

Reading as much as you can about aquarium fish and asking your dealer for advice will help you choose the best combination of fish for your size tank. Keep in mind that descriptions of fish behavior are generalizations, and some individual fish are exceptions to the rules.

When shopping for fish, the first thing to keep in mind is that the dealer's tanks are greatly overstocked. The store tanks receive much more maintenance than home aquariums do, and the fish will only be in those tanks for a relatively short time, perhaps a week or less in many cases.

Also remember that the vast majority of fish sold in stores are very young. You must stock the tank based on the normal adult size that the fish should reach when mature.

This will mean that for the first several months, the tank will appear somewhat empty. Don't fill that space with more fish! The small fish already in the tank can only grow to healthy adult size if there is room.

Healthy fish are active. Avoid fish that have poor color, fins clamped close to the body, or odd swimming behavior.

When you approach the tank, the fish should crowd to the front in anticipation of being fed. If some of the fish in the tank are clearly sick, do not purchase any fish from that tank.

When choosing fish, keep in mind that you don't want to select ones that all live in the same part of the tank. That is, you want a community in which some fish swim near the top of the tank, others in the middle, and some at the bottom. Some species tend to stay at one level most of the time, whereas others are all over the tank.

Learn how to introduce aquarium fish to their new surroundings on the next page.

To learn more about freshwater aquariums, see:


Choosing Aquarium Fish: Into the Tank

choosing fish, into the tank
Fish owners have to be cautious and careful before transitioning a new member into the tank.
© Alexander Belokurov

You should try to set aside time to make a special trip to the aquarium store when you buy your fish. If you intend to do other errands besides shopping for fish, save the aquarium store for last.

The bags of fish should be taken directly home. The small volume of water in each bag is subject to rapid temperature changes and is easily polluted, and the fish will be under considerable stress.


It has been common advice for years to float the bags in the aquarium for at least 15 minutes to equalize the temperatures of the bags with the tank water. However, because this only increases the stress on the fish, which are already under considerable stress from being netted, bagged, and transported, it is usually better to add the fish as soon as you arrive home.

The only exception is if the water in the aquarium is colder than that in the bags, in which case floating the bags will be necessary.

Some experienced hobbyists do not add the bag water to their tank to avoid the possibility of inadvertently introducing any disease-causing organisms to the aquarium. Instead, they place a large net over a bucket and pour the contents of the bag into it, and then they release the netted fish into the aquarium. Repeat this procedure for each bag and discard the water in the bucket.

Give the fish time to adjust to their new home. The tank lights should remain off for at least several hours. Some species are more prone to hiding than others, but as they become used to their surroundings they will spend more and more time in the open.

Ideally, before adding new fish to an existing tank, the new fish should spend a minimum of two weeks in a quarantine tank. This gives you time to see if they develop any diseases as a result of the stresses involved in getting them home.

If you have a sponge filter in the display tank, you can transfer it to the quarantine tank and have instant biological filtration. A heater and a hood with a light are the only other requirements.

The quarantine tank does not have to have gravel or aquascaping, but hiding places should be provided. Small, clean flowerpots broken in half and laid on their sides are excellent for this purpose.

You should also feed established residents just before you finally add the new fish to a community tank. A full stomach will make them less aggressive.

To learn more about freshwater aquariums, see: