50-70% and Safe is for the replaced volume of course.
How fast are you replacing the water?
A double dose of Safe for 150 gal is 1/4 teaspoon? I would increase it to 1.5 teaspoons.
50-70% and Safe is for the replaced volume of course.
Was it corroded? Strip back the paper or plastic and see.
It takes very little salt to make nitrite non-toxic and nitrite should make the gills brown so I wonder if something else is going on...
If it is nitrite there might be damage due to hypoxia. If not, it's 100% reversible.