I never have uncovered tanks!! Several reasons are.
Water Evaporation
Fish jump
Airborne crap can get in
Thanks for the food recommendations, but I'll stick with live blackworms until they hit about the 4" mark.
Live blackworms are healthy and pure protein which is why mine are growing so quickly!!
After they hit the 4" mark, they will be weened off the blackworms and on to shrimp and nightcrawlers.