Raw seafood from the grocery store (shrimp, scallop, clams, etc). No chemicals or additives. Buy them frozen. Then thaw in a separate bowl containing water from tank. I use tongs to pick up the food from the bowl and stick the food above the water. Less messy that way IME. Then dump the water from the bowl into the toilet.
Feed until tummy is slightly rounded. Very easy to overfeed and difficult to starve them.