Just be careful of the above product, it is advisable to syphon an any as you can 1st, to avoid a huge die off inside the tank, as some guys have lost everything from using it, just google it.
My mothers 600l system was infested by them, just regular syphoning of them off the rocks and sand, and ensuring the iodine levels where correct, seemed to help eradicate them.
I also added a Mandarin to the system, NOT TO TRY RID THE FLATWORM, but as my mother always wanted one, so not sure if it help, didn't see it eating any!