You can skip many days. I keep my fish on a really low feeding schedule. I usually feed the reef fish every three days. My puffer tank every other day. One time I went on vacation, my friend was supposed to feed the fish...he forgot and the fish didn't eat for 7 days. When I came back I thought I was going to see some distressed fish. They were just fine. The meat eating fish can go a long time without meals. The veggie guys are the ones that need to be fed more regularly. Not a good idea to let a tang,for example, go without food for a long time. The long and short of it is, keep your fish lean and your tank shall look supreme.