Why wont the white cloud in my tank clear up?

I have a 50 gal corner fish tank with 4 Oranda gold fish, 2 Pleco and one albino cat fish. the water has been cloudy for 3 months now and it does not seem to be clearing up anytime soon. all the levels are good. i don't over feed them and i try to do a 20% water change and gravel sweep once a week. i have been changing my filters about once a month and i have two filters going at the same time. one of my filters is a canister with a polishing filter in it. i have even replaced the polishing filter for a new one to see if that would help clear it up. i am at a total loss. i have asked a ton of questions and have tried different chemicals to help clear it up, i even decided to do nothing at all and let it go but its still really cloudy. pulled out all the fake plants to see if that would help the cloud settle but its still real cloudy. i fear that the only other recourse to this menacing cloud that wont go away is to just pull out the fish. clean the stones real good and just start all over. to give you an idea at how frustrated i am with it. i set up a 29 gal tank last month and the water cleared up in 3 weeks after i set it up. i did Absolutely nothing for that tank except my water changes. so now if anybody has any idea that will help me get rid of the evil white cloud in my big fish tank i would be ever so grateful. two of my oranda's are 3 inches long one is 2 inches and the other is 1.5 inches. one pleco is 5 inches and the other is one inch and my albino cat is 1 inch.