What you are seeing is called an algae bloom. These occur when there is an overabundance of nitrates or phosphates or both and they will last as long there is source if nutrients to feed them. Have you tested for nitrate? You may or not get a reading for nitrate but this doesn't mean that the biological filtration has been affected, all this means is that the algae is using them up as they are produced.
The easiest way to treat this is to do a total tank blackout. This is perfectly safe to do and won't harm the fish or any live plants that are in the tank.
Directions:
1. Feed the fish. After the fish eat perform a large water change (50% is fine) and a very thorough gravel vac. Be sure to vacuum underneath all of the decor.
2. Turn the lights off and cover the tank with a blanket, sheets, dark trash bags-anything as long as no light can enter the tank.
3. Leave the lights off for four days and no peeking. The fish will be fine for a few days without feeding.
4. After the fourth day pull back the cover and look at the water. If the water is clear perform another large water change and thorough gravel vac to remove the dead algae. The tank should be good to go at this point.
5. If the water is still cloudy then continue the blackout for a few more days.
Some things to help prevent this from happening you have already started.
- Cut back on the amount of food being fed daily
- Increase the frequency of water changes and gravel vacs to remove nitrate and phosphate from the water column
- Move the tank to a different location if it gets hit directly by sunlight
- Cut back on the amount of time the lights are on.
Live plants will also help to use up nutrients and can out compete the algae for them. This is especially true of fast growing stem plants like Hygrophila, Ambulia, Cabomba and Hornwort just to name a few.
HTH,
Bryan
