There's a few different things you can feed them!
A lot of people feed them pelleted food. There's several different brands, any will do I believe.
There's also frozen food. Brine Shrimp, Daphnia, and Bloodworms are good ones.
And some do live. I've never used that but live ones people use are Daphnia and Brine Shrimp.
There's also the freeze dried, such as bloodworms. But those are known to cause bloat so most will avoid those.
Omega One, New Life Spectrum, and Fluval Bug Bites are some of the suggested brands I see thrown around a lot, too. I‘ve always done pellets; it’s easier for me to figure out how many to give him, and I prefer that he eats the pellets as they slowly sink rather than from the surface. I prefer frozen food as well when I give him variety.