One of the more common myths about fish-keeping is that a fish will only grow to the size of the aquarium it is kept in. This mis-information likely started with Goldfish kept in bowls that did not seem to grow even though some achieved a life-span of several years (still a short life for a Goldfish). Somehow, this thinking then transferred into keeping tropical fish in aquariums and has remained in our lexicon ever since. This post aims to explain why this theory has taken hold in the mainstream hobbyist’s knowledge base, why it is incorrect thinking, and what you can do for your fish to make sure it’s life includes getting to the size it is genetically pre-destined to achieve.   

Presented here is a quick overview of factors that would limit the growth of a fish:

Nitrates
Allowing nitrates to build will not only limit the growth of the fish, but cut his/her life short as well. While keeping your nitrates at absolute zero is not always needed, keeping the levels below 25mg/l is. Even beyond that, the only hobbyist who would want to promote nitrate levels of over 15mg/l are those of us keeping planted tanks that require nitrogen as a food source for our plants. Nitrates are removed in aquariums two ways: water changes, or by plants (or both). We understand that there are colonies of bacteria that feed on nitrogen as well but this post is aimed at a reader with a more general aquarium knowledge base. Allowing your nitrates to remain high without the use of aquatic plants or water changes will lead to problems well beyond that of a growth-stunted fish. Algae blooms for example, which will take well more of your time to eradicate than simply keeping up on your water changes to begin with. In the old fashioned Goldfish bowl, filtration was rarely even considered and water changes were performed rarely if ever as most people believed that “old water” was better for the fish and that simply "topping off" the bowl of water lost from evaporation was, in fact, “refreshing” the water. You can begin to see why the fish in this case would not grow to even a fraction of it’s genetic ability.

Pheromones
Hormones given off by fish are common in the biology of nearly every animal but rarely considered as a factor of growth in our aquariums. The truth is that pheromones play a much larger part in the growth of our fish than we have ever been informed of. In a fish's world, whether it be in a lake, stream or river, the largest, most genetically fit specimen gets to mate first. In many species the largest, fittest fish gets to lead the shoal or school as well. This is especially true in shoaling cichlids like Discus and Angelfish. Being large and leading the group is something that is important to these fish so there becomes an advantage in keeping the other members of your group smaller. In an enclosed system like an aquarium, the growth inhibiting hormones that these fish release can build which affects the fish in unnatural ways that, when left unchecked, can greatly reduce quality of life and even cause death. This is a major reason why Discus keepers perform large water changes more frequently.

Diet
In the wild, not only do the fish have access to foods beyond what we have available to us as hobbyists, but they can graze throughout the day, eating only what they need when they need it. The energy spent by swimming is then replenished in a whole and natural way. In the aquarium, we all too often subject our fish to a diet that is sub-par if not flat out incorrect for the species we are keeping. This problem is compounded by keeping fish together that are not part of the same ecosystem in the wild and are adapted to eating foods quite different to that of the other fish they are now forced to live with. The foods offered to the mainstream hobbyist are sometimes drastically inadequate and full of fillers and ingredients that no fish would ever eat in nature. There are great choices now available to us as fishkeepers but it takes a source like a good store to care enough to educate the public on how alternative foods might benefit their fish. Many species of fish eat far more vegetation, for example, than what they would ever be offered in the home aquarium. These fish will take foods high in animal protein but suffer from issues of the liver more times than not because of the proteins in the diet which is not natural for them.

Water Quality
We now know that nitrates and pheromones can lead to insufficient growth in fish. When water changes are performed, we dilute these properties to levels that are at least safer for the animals. When we perform water changes we are also replenishing trace elements essential for the fish to build a proper slim coat which acts as a protective shield against countless factors that would otherwise harm the fish. Dissolved solids are also removed and carbonates are replenished keeping our ph levels from fluctuating. The amount of water needed to remove per water change and the time in between water changes is something that should be configured based on your set-up, the fish's biological load and other factors like size of the aquarium and you should consult a store that you trust to give you this information. Recommendations like a 25% monthly change across the board, is flawed advice.

Water Chemistry
This plays into our philosophy of keeping fish in biotopic conditions that closely resemble as many factors to their native waters as possible. Not only does this add an interesting and fun set of challenges, but the fish will act and grow in ways not possible otherwise. Fish kept in the wrong type of environment will live with a level of stress that will affect growth as well as other areas of its health. For example, cichlids from the Rift Valley areas of Africa need a high ph value that is rich in trace elements and a very high value of hardness to their water. Not getting these things is a major stressor on the fish to the point of death in many cases. Another example, and the opposite extreme as the African cichlids, are the dwarf cichlids of the Amazon River in northern South America. The water of this river system is very acidic with a very low ph. The dwarf cichlids from this part of the world cannot tolerate water much higher in ph and hardness than they would be on in the wild.

Genetics
Just like in humans, fish of the same species do not all grow at the same rates or achieve the same size as each other. Fish which have been bred in captivity using the same male and female for generations will often end up producing offspring considerably smaller than their wild cousins. Fish bred to express certain traits will often have the side effect of being smaller as well.

There is some recent research to suggest that a link exists between the space of an environment and growth rates in some species of fish. This just means that if you would like to see your fish grow at the natural rate and reach the saturation in color it would achieve in the wild, you will need to give your fish the space required to do so. Since we now know that large fish will not stay small in a small tank we may need to rethink the going knowledge of appropriate tank sizes for many of the species we keep, meaning bigger tanks for even the smaller fish, and much bigger tanks for the larger ones.

In conclusion, fish “growing to the size of their tank” only appears to be true because in smaller environments the water quality is more than likely not up to the standards which would allow a fish to reach its full size. Toxins build much faster in smaller tanks and have a direct effect on the fish instead of in bigger tanks, where the toxins are diluted to a greater extent and effect the fish much slower, giving you time to perform the needed water change before drastically limiting size and growth rates.