teh red planaria themselves are not really harmful, however as stated above, they can smother corals, and can also reproduce to plague proportions very rapidly....they are hoever toxic if they die off in large concentrations.....the owner of a lfs back home in pueblo had a nasty outbreak of them in his home tank, and he treated it with a vet grade dewormer used to treat horses...the name of the stuff is in Baensch's Marine Atlas vol. 1......however he used the smalles dose of the dewormer as possible, and immediately after introduction, his tank water turned yellow because ofthe massive dieoff....there were many more than he had expected in the tank, and the mass toxins they released upon die off killed every fish in his tank, but the corals were all fine! crazy stuff!
anyhow, just a bit of info for ya!
good luck
jon