Even before you’ve put a single drop into your aquarium, you are probably already dreaming of that first little fish swimming around in there. However, before you buy that pretty little blue damsel, or any other fish that has caught your eye at the store, go home without it, and research that species.​

Most beginning aquarists put a lot of planning into their tanks, equipment, water, supplies and configurations. The same thought and planning should also be put into your livestock selections. Here is a basic guide to making your choices and what you should consider.

Size

This is very important to consider in relation to your tank size. You should consider the maximum, adult size that your fish will grow to. The old wives tale that a fish will only grow to the size that a tank can support is a dangerous assumption to make.

Many tangs require a minimum tank size of 100 gallons. In many cases, tank size recommendations are made based on standard tank measurements, not necessarily on volume. A 100 gallon vertical tank will not make a tang happy, as it requires a good deal of horizontal swimming room.

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Another point to consider with the size of your fish is how small it is. A tiny clown fish is very prone to becoming mince meat in your powerhead. Tangs that are smaller than 2 inches are often too stressed to live a healthy life in your aquarium and may die at an early age. Little fish can also get sucked into your return lines, possibly causing a tank failure or a flood. Small fish also may be bullied or even eaten by larger fish, anemones, and even some corals in your system.

Temperament

Some fish will acclimate very nicely and join your reef community very peacefully. Others will try to take over, often killing other fish along the way. Likewise, some of your older and more established fish may resent any newcomer and bully it mercilessly. You must keep in mind a fish’s likely personality when choosing it. Not all individuals of a certain fish species will behave as the book says it will, but the observed behavior of the species as a whole is more likely to behave a certain way.

A clown fish, and other species in the damsel family, are generally rated as aggressive fish, not only interspecies, but also against other community residents. They are fiercely territorial and will attack other fish that approach its home. This is one example of fish behavior. Tangs, when added at different times, may attack any new tang in the tank and can kill it with its tail barb, if certain acclimation steps are not taken.

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If you are planning a reef community, you will want to plan for peaceful, or even mildly aggressive fish. Otherwise, you may have some gang wars going on in your tank.

Diet

What will your fish eat? A picky eater will not thrive as well as a garbage hound and may require special dietary care. If you have mostly herbivores (like tangs, rabbitfish, and blennies) in your tank, and then you add an omnivore or carnivore, you’ll need to have the proper food supplements on hand to meet their needs.

Some fish will only eat live foods, and none of the standard available fare of frozen, dried, or flake foods. For these fish you will need to have live food available, or a refugium stocked with pods and attached to your system. Some fish, such as sleeper gobies, are slower than others and cannot keep up with aggressive eaters. These fish may need to be fed at different times, or target fed after the others have eaten.

Fish that prey on corals or crustaceans may not be desirable in a reef community, especially if you would like to keep any of the ornamental shrimp or cleaner crabs. Fish with specialized diets, like a Copperband Butterflyfish (which relies on pest anemones), may perish once the natural food source is depleted.

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You must be able to provide the proper nutrition for the species you intend to care for. They do not have the option of swimming off YOUR reef to find better food. Poor nutrition and poor feeding habits on the part of the aquarist are responsible for many disease outbreaks and deaths.

Gender

Gender plays a good part in fish aggression, and differing size and/or coloration between the sexes may influence your decision to purchase a particular species. Some fish are very difficult to distinguish between the males and females, while others have very clear differences.

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Male Bellus Angelfish & Female Bellus Angelfish​

Some fish are hermaphroditic, changing sex if necessary. Clown fish all start as males. When two are paired, usually the larger one will become a female. Some Anthias females are also capable of changing to male if no dominant male is currently present.

Habitat

The homes in which you plan to provide you fish with is also something to keep in mind. Some fish like lots of swimming room, and therefore, not a lot of rock to get in the way. Others become very stressed if they do not have a safe hiding place to observe their environment from. Still others need to have a sandy substrate and bits of rocks and shells to build a burrow.

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If you are considering a burrowing fish, you will need to ensure that your rocks are securely stacked on the bottom of your tank. A fish digging under them can easily topple over your carefully constructed rock sculptures and can result in crushed livestock, or worse, a smashed tank.

Contrary to popular belief, clownfish do not REQUIRE an anemone to host. They can live long and happy lives without an anemone and they are in fact strongly discouraged from the beginning aquarist.

Compatibility

Will your fish play nice with the other kids at school? Some fish should NEVER be placed in the same tank as certain other fish. Compatibility can tie in with diet, temperament, size, and gender factors also. Many fish should not be kept as more than one individual, unless as a mated pair (such as clown fish) or introduced simultaneously (such as tangs).

Fish will generally be more aggressive towards others of their species, or different species that share similar coloration, body shape, habitat, or food. Some fish may overcome their differences and coexist, reasonably happy. Others will battle it out until one or both are dead.

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On the other hand, some fish are happier and healthier if kept in small schools. Anthias and Chromis, for example, rely on schooling behavior in order to feel safe in open water. If you choose to keep only one individual of these types of fish, it may become a recluse, or may become ill or weak without his posse. There are many resources for determining compatibility, such as your web forums, fish keeping books, and livestock stores (online and brick-n-mortar).

Introduction Order

The order in which you introduce your fish to your tank can help ensure a healthy and safe acclimation for your new addition. Here are some general introduction guidelines. (Please note these guidelines may not apply in all situations.)

  • Most passive fish added first to most aggressive fish added last. Aggressive fish are less likely to terrorize established passive fish.
  • Fish should be added smallest to biggest, after considering aggression. For example, a large passive foxface should be added before a smaller aggressive damsel, or, small passive chromis should be added before a large aggressive tang.
  • Same or similar species should be added at the same time, if possible. Two of the same or different tangs should be added at the same time to reduce territoriality issues.
  • Females of schooling species should be added before, or at the same time as males. If you want to add more of the same species once a small school is established, only smaller females should be added, depending on the species.
Choosing and researching your fish can be fun! If you plan carefully, you can have a healthy happy community of fish in your tank. If you have your heart set on one specific fish, try to plan your other species around its specifications (i.e., carnivore, large, semi-aggressive, rock dweller).

It is your responsibility to ensure the compatibility of your fish. Impulse buying is not the way to shop for your livestock and you may have disastrous and expensive consequences. You don’t want to put a minnow in a shark tank.

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Last edited by a moderator: Jan 17, 2014
andysgirl8800, Jun 17, 2010
#1