Oh I'm not saying that you don't water change enough, didn't mean to give that impression. Just while he's in treatment, I'd up them. Every day or every other day. While a fish has an injury to its scales, or has another condition like fungus or a bacterial infection, they're vulnerable. If the scales are damaged, it's an entry point for fungus or bacteria, so secondary infections are common after an injury.
Increased water changes not only keeps nitrates close to zero, meaning minimum stress on the fish, but also means you're removing quantities of bacteria and fungal spores that could infect the fish. Bacteria multiply quickly, but if they're being knocked back every day with big water changes, it keeps their numbers in check, while keeping the fish in the cleanest water possible, gives them a better chance of avoiding secondary infections from bacteria and fungus that are always present in fish tanks.
Not at all a criticism of your husbandry! Just standard early treatment when you spot something like this on a fish, to prevent it getting worse. I followed the same plan for a young guppy who had a horrible injury. I lifted the sponge filter out of the tank for cleaning, as normal, and she must have been caught up in it somehow, and went over the side of the tank. It tore most of her tail off
View attachment 112746My quarantine tank was already occupied, so I had to treat the whole tank, but it worked well. She could swim, but not as well as usual, so target fed her. By the next morning, she had fungus on her tail stump and on her body where she hit the stand, and she looked in a bad way. But within a few days of salt treatment and daily water changes, she'd fought off the fungus and within a week or so, I could see regrowth on her tail. She was a half grown sub adult when this happened, and now she's my largest, healthiest female, only a scar on her tail to show for this.
View attachment 112747Without the salt and daily changes, I'm sure she would have died. She just needed that little bit of help to fight off the fungus while her body repaired the damage.