Bio load refers to the amount of waste produced by the animals in your tank. Bacteria have to build up, and at some point, you cannot fit more bacteria in the tank/filtration system and you go over your bioload potentially causing a crash.
Bioload is a number of things:
tank size and actual water volume
amount and type of filtration
amount and type of coral
amount, type, and size of fish
amount and type of
CUC inverts
Say you want a fish that is 1" long. Conventional freshwater thinking is 1" of fish per gallon, but 55 of those fish would probably be too much for a common 55gallon aquarium. You can get away with overstocking a little bit if you can maintain superb water quality and keep up on your maintenance, and of course it also depends on the fish.
I have my 55 freshwater overstocked by a few inches, but I have fish that swim at all different levels of the tank, so they're not getting in each other's business and stressing each other out. I'm also running about 500
GPH of filtration on the 55 tank and do weekly water changes and change the filter pads out every other week, one from each filter (each has 2 sides, one per side, so 4 pads total).
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