Is your tank near a window? How long are the lights on a day? If you suspect this, make sure the filter is running at peak performance and do a 50% water change. The next day, do a 25% water change. Keep the tank lights off until it clears up. Wash all of your decorations in hot water and wipe down the sides of the tank. Keep doing 25% water changes while thoroughly cleaning the gravel until the water clears up. Then only run the light no more than 12 hours a day. Preferrably less than that. Maybe 8-10 hours, if possible.
Daniel