The general rule is that a fish's stomach (one of the popular varieties in the pet trade) is only the size of its eye. So, use that as a guideline. I also fast my fish regularly.
I feed mine once a day, and I skip one to two days a week. Truth is that fish spend a lot of their time grazing during the day in the wild, so having them try to find food in the tank gives them some more realistic existence, plus it cuts down on the algae. Also, having real plants helps in giving them something else to eat.
Feeding is something that a lot of fishkeepers will disagree slightly about, but the general rule is that the fish will always try to eat as much as they can, but they generally need far less than we give them. The fish can easily go an entire week without ANY food, so if you are going on a vacation for a few days don't fret about the fish. And don't overfeed them before or after. They'll be fine. Try it sometime when you are home. Don't feed them for 3 or even 4 days. Their activity level won't drop, they'll still be just as active as ever. But, don't invest in those delay feeders. They are a waste of money and they add nasty stuff to your tank. If you are going away longer than a few days (more than 5 in my experience) have someone you trust stop by and feed them a pre-measured amount of food once or twice while you are away, whatever fits in their schedule. For the pre-measured amount of food, I just use a weekly prescription pill holder. It makes it foolproof, so that ANYONE can do it, even if they don't know the first thing about fish!
