Cleaning a fish tank?

His tank is far too small for 1 goldfish let alone 3! He needs to get a bigger tank or give the fish away because they will die. Either by the horrible water quality or the tank stunting their growth. A single fancy goldfish should have 20 gallons (75l) with double filtration. uming those are fancies you'd need a total or at least 50 gallons. Commons require 75 gallons each. His tank is a mere 5 gallons which is only really big enough for a betta.