Interesting. I've had those SH since they were 8 inches and never added salt. In the belief that salt helps relieves stress on "any" fish, I added, not a full dose, but a very small amount. The symptons I first noticed was the body of the fish. The tail end is just floating to the top even though the fish's head is in it's normal placement. The others did not get the same sympton, but noticed a slight irritation, in that, the fish was trying to shake something off it's skin. The other and largest fish, ended up with fungus all over it's body. In my experience, I would not do this again. Better safe than sorry. There may be some snakeheads who has higher tolerance in salt. As far as impurity, this was a used box that I used for my larger Dats tank.
I just won't make the same mistake twice.