I used to have black skirt tetras and glass cats. The tetras were way too quick for the cats so I fed them by holding the flake with two fingers and feeding them by hand when they swam around the surface.
I have a Red Tailed Shark (Similar to a Rainbow Shark) I feed them algae wafers, pea, zucchini slices, bloodworms, and tetra color sinking granuals. Basically anything that sinks to the bottom. They especially it especially likes cleaning algae off the glass and plants.