"Overstocking" is a term used when keeping fish. I've heard it used when someone has too many fish, but I've also heard it suggested as a way to make semi-aggressive fish more timid.
So, is overstocking an aquarium good or bad? What does it mean?
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Overstocking a fish tank means putting more fish in the tank than the tank can handle. There are several factors to consider with how many fish can fit in a tank, including the bacteria that converts the chemicals (ammonia from fish waste, to nitrites, to nitrates) being able to keep up, the amount of oxygen being put into the tank vs the amount being used up, the filter being able to take out enough waste, and just plain swimming room.
The term is commonly used for the extreme, where someone puts too many fish in a tank, or perhaps to large of fish in a tank. But overstocking a fish tank isn't necessarily a bad thing. It just means that you will have to be more careful in monitoring the water quality. Changing the filters often, and performing regular water changes to keep up with the buildup of nitrates.
With some of the schooling fish that are semi-aggressive, it's suggested that you overstock the tank so that none of the fish feel like they have a territory that they can call their own. If they don't feel like they have any territory in the tank, then they're less likely to bully the other fish in the tank in defense of their territory.
It's important to note that overstocking the fish tank might not be the first solution, as a fish tank should really only be overstocked if you're committed to making sure the water stays clean.
If you only have a couple fish of a species, and they're picking on other fish in the tank, it could be that they're just looking for more members in their school and don't know how to behave with the other fish (different species of fish school differently). So simply getting a few more of that species of fish could give them their own school to be happy with.
Generally with more aggressive fish, the larger the school the less likely they'll pick on the other fish in the tank. So if you have room, you might want to try adding some more to their school. Tiger barbs for example, might remain fairly aggressive until there's about 8-10 of them in a tank.