There are these things that you can get from just about any store (I think) that will feed your fish for a number of days, but the biggest I've seen was fourteen days. (They're feeding blocks, I think they're called. Mine came in cool shapes, such as fish or clams.) They can make a tank really dirty though, from my experience. I'm not really too sure on how healthy they are for your fish, either. Sometimes my fish would die soon after putting it in.