I have one fresh water aquarium. It is 25 litres, so about 20 gallons. I have black neon tetras, neon tetras, and serpae tetras. They are the easiest fish for me to keep. I like many, many fish in the tank, so I stick with varieties which are small as adults, so I can get lots in there! Right now I only hav about 6, often I have as many as 16.