there are good and bad flat worms... Are you 100% sure its a FW
? Read and follow the instruction on the bottle. A few of my friends have used it and it works well but like Hara says have plenty of fresh SW
ready to go and carbon on hand.
The problem with the dead FW
is they release toxins when dead that can be harmful to the tank. So syphon out all the FW
you can get before nuking the tank and all the dead ones after.
My friend put a bright light in the front of his tank to draw them out, they love light and need it to live. If you have any sensitive inverts I would take them out.. My frined killed his seahare and I want to say a cucumber also using it..