i found that the LEAST reliable automatic fish feeder is a neighbor.. i went out to sea for a month, and left my roommate to feed the fish.. i came back and he had used almost the ENTIRE canister (the biggest can of tetramin they make) and there was almost 2" of detrius and mung in the bottom.. on the upside, even though that had happened, my guppy population had quadrupled, and the plants i had in there had completely taken over.
automatic feeders are a waste of $$ IMO since fish can go quite a while without food.