Neale Monks advises a PFK reader on the causes of "stunting" in the aquarium.
Q. I am aware that some species of fish produce a growth inhibiting hormone that can have marked effects in a densely stocked aquarium.
Is this hormone removed from the water by filtering it through carbon?
Peter Russell, email
A. You are correct that there is some evidence that chemicals produced by fish as they grow can suppress the growth of other fish of the same species in their immediate environment. This is primarily an issue for fish farmers, and I believe most of the evidence for this phenomenon comes from studies of cyprinids (such as carp) and salmonids (such as trout).
It's important to note that some fish clearly don't work this way, for example Tilapia (cichlids farmed as food) can grow to large sizes seemingly regardless of stocking density provided water quality is adequate. Aquarists will also be aware that catfish like plecs, Channel catfish and Iridescent catfish also seem to reach large sizes, even in overstocked tanks.
In any case, while chemicals produced by growing fish may be responsible for "stunting" under some circumstances, other factors may be at play as well. An overcrowded tank is an unhealthy environment, and unhealthy fish won't grow as well as healthy fish.
High nitrate levels for example are often characteristic of overstocked tanks, and even if not immediately toxic, nitrate does have an appreciable effect on the health of your fish. Fish grow continually through their lives, but they grow more quickly when young, so if kept in an aquarium that's too small for the first year or two, a fish may never reach its largest size no matter how well it's kept thereafter.
You can often see this with Clown loaches, a species that rarely reaches its full size in captivity because so few people keep them in adequately large aquaria as youngsters, let alone as adults.
So while (fresh) carbon should remove hormones from the water, aquarium size and water changes are factors that affect nitrate level. If you want a fish to reach its full potential, the key thing is to keep it in a clean, spacious tank, and not to rely on chemical media to remove dissolved metabolites or hormones.
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