If you're really worried about the calcium, give them a couple fruit flies dusted with calcium powder. Also daphina, moina (a small relative of daphina), and fairy shrimp (and maybe clam shrimp) would be good foods. Daphina and moina can be cultured by keeping a bin outside filled with cheap plants like elodea out in the sun to get the water nice and green, feed them yeast a few times a week and they should be good. Fairy shrimp are just brine shrimp except can live in freshwater for a few months and can be cultured by adding eggs to an aquarium, filling it up, and once they lay new eggs and die you drain the water, let it dry, wait a few months, then repeat the process. Fairy shrimp can eat yeast and fish flakes.