7. Common Goldfish
Common (single tailed) goldfish are more streamline, energetic, and larger than fancy goldfish varieties. Due to their size, energy, and waste production, you need at least 40 gallons per single common goldfish. Given that common goldfish do best with other common goldfish, you will realistically be looking at aquariums that are 80 gallons or more. In fact, many people move common goldfish to outdoor ponds to best see them grow and thrive.