Because the fish is a butterfly, the fins are somewhat tender and lead ray is easily broken and that is what it would appear to be. I would treat this with Tricide Neo as a topical, applying some of the dry powder with a Q-tip to the reddened area. This will require catching and sedating with Clove Oil or MS222, then cleaning with hydrogen peroxide or iodine, and then applying the powder. Open the packet of Tricide Neo and dump a small amount out onto a plastic bag, then with a wetted Q-tip, coat the Q-tip and then rub the reddened area. Give it a couple of minutes before placing in clean water to revive and then put back in the pond.
Salt has some uses, but in a planted facility, it has no use. Some plants will tolerate up to 0.2% salt, but the needed salinity for most applications is 0.3 to 0.6% which will kill most plants.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard