I just put a power head in my tank and later i noticed it not working a pulled it out a 2 fish had died while bein suckd into it and 1 of them was a real good 1. Trial and error.
i've just added a canister to the 4ft and it sucks a fair old amount in, but the little strainer u get just clogs with plant matter from the tinfoil barb, so i swapped it for a big block of foam so there's no major suck now and the foam catches all the big stuff so the filter don't clog up so quick either