That's good advice. You should never over crowd an aquarium, so species selection depends quite a bit on the adult size of the fish you choose. Male Bettas are very colorful, and a single one can do well in a small aquarium provided the tank is carefully maintained. If the aquarium is ten gallons or larger, you may want to consider other species. If you're just starting in the hobby, try some of the more common, less expensive fish and plants. We all make mistakes getting started, and cheap mistakes are better than expensive ones.