You cannot treat the whitespot on the fish, its under the fish scales or on the gills. The ones under scales is within the protective slime coating. So unless you got something that can penetrate the slime, you will not be able to treat it on the fish.
When it falls off, you got a few hours to treat it. Once it reach the substrate and encysts nothing can touch it. Except hypo salinity that breaks down a big percentage of the cysts. But some do survive. Hatching and you have another 8 to 18 hours to kill the parasite. BEFORE it can reach the host.
Then WS is clever. They fall of mostly at night, before midnight. So they fall off where the fish sleeps. And they hatch just before dawn. Where the fish sleeps. That helps them in being able to find a host.
The goal is to remove the WS from the fish. But you cannot treat the fish in the display environment.
My opinion, the only solution is to remove the environment from the fish.