I have a small 2 and a half gallon tank. What fish are good to put in this tank without being over crowded? I have been to the pet store over 3 times in the last two days because my fish have either been too aggressive or it just died. I don't know what to put in my tank. Right now I have a small pregnant black molly and then a balloon molly wich is going back to the store tomorrow because she killed my other balloon molly... . Please help...