Hi Todd,
You've been finding all sorts out the hard way. A big thing I've learned is to buy from a local pet store that has people who are interested in the hobby also. They will help you choose fish that go together well, and often have healthier fish. A lot of newbies buy from Wal mart, and not that ALL walmarts are bad, but it seems they don't really know how to take care of thier tanks to begin with. When you're buying fish ANYWHERE, look at the other fish in the tank. Don't buy if the fish are pale, have fins or tails that seem frayed, or have white spots, even the tiniest white spots. You usually get to choose. Go for the fat active ones, but not if they're too agressive. Hope that helps a bit. also, don't buy if the tank has black algea in it. It's near impossible to get rid of. Good Luck!