A look at the inside of a Resun AF-2003 fish feeder.

This is a fairly cheap automatic fish feeder bought on ebay. It has a detachable drum that is filled with fish food and you set a switch at the back for either 12 hour or 24 hour cycles. When it is time to dispense the fish food the drum rotates a single turn. As it turns some of the food falls into an adjustable port at the side of the drum and is then dumped into the water.
Standby current is about 0.25mA (250uA) which should give a good battery life.
The plastic gears are very noisy when it does rotate, but I guess that could be useful for letting you know it's doing it's job.
The unit comes with velcro pads and a clamp system for mounting it to your tank.
I don't actually have a fish tank. The reason I bought this was to evaluate it for use in a public art project, part of which involved the daily dumping of a small amount of glitter on a pedestrian walkway.