Okay I confess - I have ten neon tetras in a 20 litre (6 US Gallons) tank. It is a rectangle shape so has plenty of surface area and swimming space. It has a top notch filtration system and heater and I am careful not to overfeed them so they don't make too much waste. They seem very happy and have bright colours and normal behaviour. They have lots of plants and dark stones to hide in. I really, really love them and I am concerned because everyone says 10 neons should have 10 gallons. Is this so they have lots of swimming space or because of the build up of ammonia in the water? I will be gutted if any get sick or die!