It looks a lot like a bacteria bloom, which would be caused from the overstocking issue and overfeeding. My tank just went through one. The water got really milky white.
If the water chemistry is fine, then its more of a visual issue. The bacteria wont hurt the fish. They will consume a lot of oxygen though. The only thing you can do, if it is a bacteria bloom, is decrease the amount of organic matter and wait it out. Cutting back the amount you feed them, and decreasing the bioload should help. Keep an eye on the chemistry. If anything starts to spike do a water change, but if the chemistry is normal, leave it alone. Adding water into the tank actually provides new nutrients and space for the bacteria to grow, continuing the problem.