They eat ALGAE!!!!! helce the name ALGAE EATER!!! lol! I there is no algae in your tank for them to eat, feed the algae eaters algae pellets, which you can find at your local fish store :]
If you mean Plecos I have read they especially like romaine lettuce (whatever that is! must find out) other fish might eat it too. Also most fish like cooked peas with the skin removed. Don't give them too many peas or too often as it makes them "go" too much. Peas sink on their own IME.