esshup, I have never treated an entire pond. My fear/concern with this is each species and size of each species -as you well know-can react differently long-term.
I have several concerns on treating the entire pond for a couple of other reasons with one being a lower concentration of salt does little to certain types of fungus if concentration is not "hot" enough to extract life supporting water/moisture from the fungus to actually end it's cycle. The salts have to be high enough concentration to also stimulate slime coat production, which in my experience, is only achievable in a more controlled environment such as the tank method using a complete saturation to a point the fish lose equilibrium and are carefully monitored through that process to ensure the time period is such they are removed to fresh water where they slowly regain that equilibrium, right themselves and color fully returns. The fungus spores are completely sucked dry and the enhanced production of slime coat immediately protects the areas affected.
I will add this also cleans gill flukes and increases gill function very quickly.
In lower concentrations, I have no experience, documents or data to provide conclusive evidence that treating a pond with salt will "fix" this situation.
I feel strongly this action "could" adversely affect the eco system in other ways that may not show up or even be recognized.
We treat all fish collected by net samples-whether gill nets or trap nets in this manner, I have used this for 2 years now on my personal pond during angling and netting and have yet to kill a fish.
My personal recommendation would be to utilize Bob Lusk's recommendations of mixing neomycin in fish food, mixing well and treating until condition disappears.
I'm not comfortable with any other method to actually treat the entire pond, but there well may be others.
