Formalin is fine in a planted tank, will actually kill every bit of algae present also. Formalin is usually used for bacterial infections and flukes, clout is used for external parasites, they can be more effective used together. Dose formalin according to instructions on package, usually 1 ml per 10 gallons; keep temps down when using formalin (it depletes O2 in water), 50% water change 8 hours after treatment (this is very important, I killed several Discus ignoring this), treat every other day for 2-3 weeks to kill all egg hatchlings (again a lesson learned). After treatment raise temp to 90F, if you can catch the fish without stressing it too much, give it a salt bath. The higher temps will raise metabolism and get em munchin', the salt bath will make the fish feel better, help heal any lesions, and help get the slime coat back. You can put salt in the tank water but be careful, too much of it WILL hurt your plants.......DC