Returning fish with brook to tank

[dantodd]

I'm treating my fish with formalin dips for brook.

I am worried that the brook will re-appear once the fish go back in the tank. Does formalin give them prophylactic protection long enough for the brook parasites die since they will be host-less?

Do I need to pull all the fish out of the tank and let it sit fallow for a period of time? This would be a problem since I'd need 3 holding tanks and only have one.

Thanks for the help.