if you want to help limit their numbers, decrease your feeding and increase your gravel vacs. They can also be squished and fed to fish as a treat. You can also use a piece of vegetable as bait to pick them out in greater numbers. I like them, i think they are a good addition to a tank to help as clean up, but when they get to numerous often its necessary to thin their numbers.
What she said.... I encourage them intentionally though. They clean my breeding tanks and provide treats for my loaches.