have you actually checked the ammonia and nitrite? Are they both zero and no higher? If this is the case, I don't think you will benefit from adding something like stress zyme.occasionally, you can get cloudy water develop. Have you given the gravel a clean? Occasionally when you clean gravel, nutrients can be released into the water which can make the water go cloudy. I would suggest that you keep testing the water for ammonia and nitrites. I would suggest that you feed extremely sparingly until the water is completely clear. Without knowing exactly what's happened to your tank, it's very difficult to establish why this has happened. Occasionally when people add new fish, the water can go cloudy because the filter has to catch up.
So, if you could tell us your ammonia and nitrite readings, we can go from there. If you do have ammonia nitrite present, just do a small water change every day until things go back to normal. It might take a few days for the cloudiness to dissipate.