I have had very good experiences treating planaria with Panacur KH 250. It's easy and safe for the rest of the tank's inhabitants. You only need one Panacur tablet.
The following treatment works for tanks with about 50 to 150 liter.
If possible, it is advisable to vacuum the substrate before treatment and clean your filter. There must be sufficient oxygen levels in your tank.
Crush about a quarter of the tablet as finely as possible and add the whole thing to the aquarium.
After 2 days at the latest, most of the planarias are dead. The rest are very weak. You should vacuum the substrate again and start again with the Panacur treatment. After 2 more days no planaria should be there.
To be save repeat the treatment after about 3 to 4 weeks.