There are loads of things you could feed them on. Krill, bloodworm, shrimp, see what your local fish store have in their freezer cabinet. You can also find food in your own garden, crickets, worms, any little insect really, it'll be very similar to what they find in their natural environment. The only thing you got to be careful of is if you use any kind of insecticides in your garden. you certainly don't want to be giving insects to your fish have they been feasting on any kind of poison and haven't yet died.