I can relate to you alot, i had the same exact problem with my 55 gallon. I have 8 tanks in all and they are all crystal clear, but my 55 was very cloudy. I tried everything to clear it, but nothing worked at all. Was making me nuts, but i finally now have it clear. I was using filter cartridges made by top fin, and it just would not clear for me. I finally after trying everything else, went back to whisper brand filters. I took my entire hang on back filter off the tank, and washed it all out again like new, added the new filter cartridges in, and witin 24 hours it was clear. I dont want to put down on top fin brand, but i personally will stay with the Whisper brand from now on. I dont know if this could help you, but it helped me. Good luck and let us know.
Sandy