I have approx. 10. They are really fun to watch and are all over the tank. If buried, they pop up instantly when food is dropped in. They climb the walls as well, but stop at the top of the water, and usually fall back down. The only negative is that they are bottom feeders, and they are part of the clean up crew. They are so fast, that if you have a weak fish/shrimp/etc. and you are not there to rescue, they have him before you even know about it.