Why are my fish dying?

I have a 20 gallon freshwater tank. I started with with 2 dalmatian mollies, 2 orange Mickey mollies, 2 orange platys, 3 silver skirt tetras, 4 fancy guppies, 1 albino Cory catfish, 2 female bettas, 1 white Molly, several mystery snails, 2 African dwarf frogs, 1 Pleco, and one other bottom feeder I don't know the name of. Of all I am left with the frogs, catfish, guppies, bettas, the unknown bottom feeder, white Molly, 1 orange Molly, and the tetras! (I traded the Pleco because of his size.) the other fish/snails died without cause in the last month. I would just find them either floating or at the bottom of the tank. Temp is between 72 and 80 degrees at all times. Ph is 6.5. Filter gets changed monthly. I even have an airstone. They get fed bloodworms, brine shrimp, and tropical crisp flakes, and a couple times here and there I'll throw in some algea wafers. Help!!! Can't seem to understand why my fish are dying and why the snails all died. And the fish are all passive. None show signs of bitten fins or signs of illness. The fish that died did all have discoloration when I found them. One more note. I have had this setup for approx. 8 months no problem.