Follow instructions right on the package..
Directions:
For the treatment of praziquantel-susceptible disease conditions in freshwater and marine aquariums. NOTE: Not recommedend for reef aquariums with Feather Duster Worms or other ornamental worms.
Start treatment with as large a water change as practical or start with new water. Any water used should first be conditioned to remove ammonia, chlorine and chloramines. Do not stop filtration, but remove activated carbon and stop foam-fractionation (protein skimming) and UV sterilization.
Shake vigorously before use. Measure at the rate of one (1) teaspoon per 20 gallons of water to be treated (One (1) fl. oz. per 120 gallons). This produces a concentration of 2.5 mg/L. Distribute the proper amount around the edge of the aquarium or directly into the filter box to achieve the best overall distribution.
A single treatment lasting 5-7 days is normally sufficient. Repeat as necessary, but no more than once every 3 to 5 days.