Hypothetically...

[Tokis-Phoenix]

If you overstock your tank then the filter/s will get clogged up a lot more quickly. Although the filter is full of decomposing muck which the beneficial bacteria thrives on, if the muck clogs up the filter too much then less oxygen will be suplied to the filter bacteria and thus less of it will grow and thrive in the filter, reducing the biological filtration of the filter.

The main reason why you shouldn't overstock your tank should be for the sake of your fish though, there are many issues that can arise when one overstocks their tank i.e. fish may become stressed, desease or parasite outbreaks have a much higher risk/chance of happening and will be much more difficult to treat, tank cleaning regimes will have to become longer due to larger water changes and longer substrate cleaning sessions and will have to be done more often, fish may breed out of control or show abnormal behavior due to crowded conditions, fish may experiences things like stunted growth etc etc.

I think how much you should under-stock or can over-stock a tank depends mostly on the exact types fish you have and their sizes and genders, the dimensions of the tank and the filtration and decor you have in it like planting etc.

 

[gilaesther]

I think how much you should under-stock or can over-stock a tank depends mostly on the exact types fish you have and their sizes and genders, the dimensions of the tank and the filtration and decor you have in it like planting etc.

That's reasonable. I did mention swimming space, I had meant that to take into account enough space that fish wouldn't be concerned for their safety, as well as enough swimming space for their swimming needs. You are correct about taking into account fish genders, types and numbers - I had assumed my idea would be something only an experienced fishkeeper would use; however, it is true that the ones that really overstock are the novices, and probably won't maintain their tank(s) properly in order to make it work.