Need Help Already!

OK . . . one of my assignments in my Science Methods class is to prove I can keep an animal alive for at least one week. Unfortunately, mammals do not count. Our teacher has giant hissing cockroaches to give us. After everyone freaking out, she told us that if we want to buy a fish on our own and keep it alive for a week, we can. So I am buying a fish. But I've never had fish. What's the cheapest, easiest-to-keep-alive fish at Wal-Mart? =] Thanx!!