I think it's a matter of luck whether you get one that eats fish food other than the "things" on the skins of fish. Based on my experience, cleaner shrimp, is quite effective in "cleaning" the fish initially, however, after sometimes, it gets a bit lazy and stops working on the fish unless the fish is very near them and remains stationary. Doctor fish, however, will follow the fish wherever it goes and cleans the stuff off the fish. They are quite persistent in doing it. They are quite clever to hide during the night and appeared only during feeding time. If you can get one that feed on fish food, it should have no problem surviving for a very long time.... i couldn't imagine what would happen to my tangs without them....