[graveyardworm]

They dont really swarm. They reproduce through fission so 1=2, 2=4, 4=8, and so on. If they're in a preferable place they dont stray very far.

There is only one true natural predator of these found in the hobby, Chelidonura Varians, also called Velvet nudibranch although not a true nudi. Generally what happens with the CV is it will eat most of them and then starve leaving some to repopulate. The other fish predators mentioned are hit or miss, usually miss. The toxic nature of the FW makes them undesireable for most fish.

Siphoning them out is truly the best method for removal as much of a pain as it might be. Like I said earlier if you have a large population you need to get them out. There are plenty of stories here on RC where they all of a sudden die and nuke the tank inhabitants. Also if you decide to treat the tank with Flatworm Exit you will have to first get the poulation as low as possible otherwise killing them will nuke your tank.