I hav my 78 gallon tank with two oscars. An undergravel filter and a internal power filter in it. I change my water 60% every week. Still water quality goes down by 4th or 5th day. Its away from direct sunlight.I never over feed. Thinking of buy a good canister filter.so please suggest which canister filter is good for a 78g tank? How much gph I need to keep my tank crystal clear?