I have six in a 20 gal
as well and I do see them quite a bit. Often when I turn on the lights in the morning they go crazy, zipping every which way, which is always fun
They definitely feed the most at night, so drop pellets in for them when you turn off the light. It's important to have a smooth substrate so they can burrow like sand or small smooth stones. There are a few empty ramshorn snail shells in their tank from time to time, but I can't say for sure who is responsible. If they do eat snails, it isn't many.