If fish are correctly fed there is no reason to fast them one or two days a week, although many will recommend it. Fish have no stomach as such, the food is digested as it passes through the gut on it's way to the anus. While they seem to take food every time you feed them, (feeding frenzy), partially digested food is passed out. This then starts to break down and decay in the tank, producing excessive load on the bio system.
That said, when ever we used to go on holiday, a few days or a week, I never used auto feeders or got a friend to call in and feed them. There was no need. All my tanks were heavily planted and if the fish got hungry enough they would nibble on the plants and algae. When we returned the water always appeared to "shine" it was that clean, and the fish looked as healthy as could be.
