Under feeding is better than over feeding. Once a day is fine, and not too much, or you'll have to change the water more. If you skip a day don't worry, just continue regularly the next day.

Fish will typically eat like horses, as in they'll eat until their stomachs pretty much explode, they have no concept of leaving some. This is why I say under feeding is better. I feed my fish twice a day (most people say 3) and they're fine. Since it is one fish, once a day will work.