Cloudy tank?

i have a 30 gl cube tk with two shubkin goldfish one is 2.5 inch long and the other is 5inches long. it is a well established tank (2 years) and every thing was great untill recently. i am now haveing problems keeping the tank clear. 2 months ago i added an additional filter and per advise started doing 10 percent water changes every other day this was to go on till the tank cleared. after a month of this i got frustrated and did a 50 % change and completely sterlized the older filter. for a month the tank stayed clear. its NOW back to being Cloudy agian. i have two filters on the tank. one a Whisper with Bio sponge rated for a 30-60 gallon tank and the other is an Enhim (?) rated for a 50 gallon tank with 2 bio sponges and other media. i have done a 20 percent water change, changed the filter media in the whisper filter and carbon in both and no change its still cloudy, the water test with no ammonia,or nitrites. are my fish to big for that tank? what can i do to keep the water clear