Yes, I fast my fish. The day before a fast, I feed them something very high in fiber, like peas or daphnia, then they fast the next day. It's like cleaning the pipes out and then the next day they get a high protein food, like bloodworms or brine shrimp, so there's something to look forward to. This also helps keep the snail and planaria population in my planted tanks down. Fish can go for up to a week without food. Not that this is great to practice, nor have I tried this, but just some information in case you need to know.