Overstocked? What to add?

i would suggest you keep SOME fry and don't get more fish. it could help with water conditions if you add live plants. you should clean half the filter weekly and do 1 third of water change weekly if you would like to keep all the fry. it would be better if you change as much water as possible without stressing the fish. i'm pretty sure that would help in the future when your fish drop.