Your tank is too small (as you know already, as you're planning an upgrade). Your filtration is WAY too small. You need a filter rated for at least 300 gallons per hour. Nitrates don't go away without water changes. Please research the nitrogen cycle, or your fish will become gradually more and more stressed out, and then they will get sick, and eventually, they'll die. You NEED to know this information in order to keep fish. Look, I'm not trying to be rude, but what the people on this forum do is an art. It's not as easy as "add some water, add some fish." People who think it is kill their fish. It takes more research, more thought, and more effort than what you've been giving. Buy some water tests, and learn how to use them. That way, you can test once per week, at least. Tilapia is a fish. They sell it at the grocery store. Look in the seafood part of the big freezers. It comes in a bag, as do a lot of other fillets that are way better for your fish than beefheart. Please, please research. And red-tailed catfish are not cichlids, they are catfish. Cichlids are cichlids, and catfish are catfish.