Many fish can tolerate it, an equal number may take it poorly, dependent of the species and a variety of other factors. Formalin itself is quite unpredictable when combined with other major stressors.
My question would be this: if you have accurately diagnosed the pathogen and started a hyposalinity treatment at a salinity and duration appropriate for the problem you are trying to solve, why double the stress on the animal? ...kind of like baking cookies at 700 degrees instead of 350 to get them done in 1/2 the time, somethings going to get burned
