That white patch on his head isn't fin rot - looks like an injury of some kind that is leaving scarring, perhaps? Is there a guard on his heater? Any decor that he uses feel rough to the touch? Wondering if he's scraped himself against something like that cave - sometimes in the manufacturing process those things leave rough edges, feel any decor over carefully, and remove any that feel anything but smooth. Sometimes you can sand the decor down. But it almost looks like a heater burn that damaged scales. Another possibility is a tumour of some kind, has it been growing slowly? It doesn't quite look right for that though, more like an injury that hasn't healed well -
@GaryE , what do you think?
Hard to spot fin rot on a crowntail - can you try to take more photos, please? Looks like perhaps the fin beneath his body is affected, but really hard to see from that photo. Bear in mind that new growth on fins can look transparent, or looks whiteish, so sometimes people mistake fin re-growth for something bad. But really need more photos.
Salt treatment isn't a bad idea - bear in mind to still keep the tank and water clean, keep up with water changes, but replace the amount of salt that you would have removed. So if you remove 5 gallons, add another two teaspoons of aquarium salt/rock salt (not ordinary tablesalt!!) dissolved into the new water.