In my daughter's new 75 gallon aquarium, we have 4 Angelfish, 3 albino cories, 3 panda cories, 4 peppered cories, and 1 bristlenose pleco, all small/juvenile fish. We recently changed most of the gravel to sand. Everything seemed fine for a week and then the water started getting cloudy and he cories started mostly hiding in a fake stump cave. I did about a 20% water change 3 days ago. I thought I was feeding them too much (a pinch of flakes, 6 small shrimp pellets, and a pleco wafer twice a day). So I started feeding only once a day and started reading some forums. I ordered the API master test kit and tested the water today. PH-7.6, ammonia-1, nitrite-0, nitrate-0. I did another 15% water change today. I'm worried that by only feeding once a day, the angelfish will eat most of the food, but I want to get the water parameters right. Any suggestions? Sorry for such a long post. Filter is a Penguin biowheel 350, 2 airstone strips, Topfin water conditioner when we change the water.