Everything edible.
Seriously, mine will eat anything, or at least sample anything. He doesn't much care for veggies, though he'll pluck away once or twice at seaweed. He'll eat sinking wafers, algae pellets, bloodworms, brine shrimp, he'll probably eat krill if I ever try it. Last night he did his darn best to finish off the tetra pellets that hadn't sunk from the surface yet. I thought he was gasping for oxygen, until I realized he was sucking in the little pellets that were floating on the surface. There's a reason I named him Big Pig.