Canister filters were originally designed to provide biological, mechanical, and chemical filtration; however, times have changed and primarily used for chemical media and added water flow. The chemical media I speak of in carbon and purigen traps organics and help water clarity. PO4
stands for Phosphate, which can fuel unwanted algae amongst other things and there are medias that can bind it. If your tank is not drilled for an overflow, i.e. no sump, then you would have to use a hang-on-back skimmer, which simply hangs on the back of your tank.
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