I have 3 fan tail goldies and they are filthy little guys! You are probably going to need a larger filter specially if you want an algae eater in there too. An algea eater will outgrow a 20 gallon tank in no time. You don't want to over populate your take since it will just crash and you will end up with some dead fish. I have a 30 gallon tank right now that has a 90GPH canister on it. When if comes to gold fish you will want to over do it on the filtration.