They are filter feeders. The filter food from the water column with the "fan" type things on the end of there front legs. They like a gentle current and you will probably notice they will sit around a filter intake with there filters out. Can grow to a decent size, 4"-5".
They do best in a well-established tank (not just cycled) because of their style of feeding. However, they will also eat flakes off the bottom of the tank if hungry.