I have a 55g. stocked with about 13 various Cichlids. I had the same problem at one time with water getting very cloudy. Even after doing 20% water changes and nothing happened, I tried 50% water change. It cleared up for a couple hours, then back to the cloud. The only thing that I have found to clear cloudiness is vacuuming the gravel.
Now I'm not sure how many fish you have but I know that with my tank, there tends to be alot of crap (literally) getting below all the gravel. When it stays down there too long there will be a constant cloud in your water. Dont vacuum all of it though, you actually want to keep some of it in there. And once you vacuum, you will clearly be able to see whats clouding your water
