u wouldnt need one for any sized tank, unless u really wanted to have it. Basically wat a fuge does is convert ammonia and nitrites to nitrates. THen the macro algae uses these nitrates. SO this means that the tank is more stable now and that the bioload has been decreased so that u can add more things. Another think that it does is let copepods and other small creatures breed safely without teh worry of other fish to eat them. So it is not a neccesity, but its better to have it then not have it. SO to answer ur question, go get any sized tank that ur heart desires.