It could be due to overfeeding as previously stated. Or...is the tank in direct sunlight? This will cause a bacteria bloom. A bacteria bloom will also occur if it's a new tank and set up.
Sorry - just saw title - d'oh! I had this problem before with my tank - I was feeding them too much and they had fin rot. Got treatment for that, changed their food to one that says it leaves the water clearer and checked the pumps and filters etc. Has your tank got a light/fliters/pump etc?