How Many Fish Can I Add to My Aquarium?

This is one of those frequently asked questions by aquarium owners that is difficult to answer… because the answer is not usually the one that they want to hear.

“The Rule”

The hobbyist community has “the rule,” and then countless successful exceptions (and I would guess countless unsuccessful attempts, as well).

The rule hovers around adding only one or two fish, and then waiting for two weeks before adding more. There is a lot of reason and science behind this rule, and much of it depends on the tank and its condition.

The thought behind the rule is that an aquarium has “x” amount of waste-processing bacteria which take toxins from decaying food and waste and converts them to less toxic compounds. Adding one or two fish adds more food and waste, but the bacteria can still process it, and will grow to be able to handle it better and more efficiently. After two weeks,it should be able to handle another fish, or two.

Adding too many fish at one time will mean that there is too much waste for the bacteria to process, and will therefore go unprocessed, and will turn into toxic decay.

“The Exception”

Rules have exceptions, including this one. But successfully doing it usually takes a lot of care and attention by the hobbyist.

The exception that I have seen, with mixed results, is to add several fish in waves. The rationale behind it is that if just one fish goes in, it can become the runt of the tank very quickly, sometimes even bullied to death. However, by adding that many fish at one time, the entire tank has to reestablish its hierarchy.

I don’t recall seeing this approach have 100% success. There are at least a couple fish that do not make it. However, many who add fish in waves will tell you that the fish that did not make it wouldn’t have made it as a single addition, either.

However, the fish that do make it are strong, and the aquarium usually ends up with a balanced community.

It is a controversial subject about which I have seen heated opinions expressed from several points of view. My normal approach is to only add a couple at a time, unless the customer wants me to do more after I have shared this information with them, or if it is a group of shoaling fish like anthias or cardinals. It just seems to be the most prudent way to walk down what can be a sensitive path.

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