I don't believe it's fin rot-
I do want to recommend something I saw that you said in your original post that was missed-
Your cleaning schedule is on the very low end- giving the bacteria that rot needs the chance to grow.
In a 5 gal, filtered, cycled tank with live plants- 25-30% water changes per week is needed, with once to twice monthly siphoning of the gravel. This is assuming that you have multiple live plants and not just one- but once every couple of weeks really isn't idea.
You can use the 1 gallon pre-mix for multiple days- just keep it at room temp and shake it before each use. I personally wouldn't use it, but if you decide to do so- 1 tsp per gallon, make sure you dissolve the salt prior to adding your guy in it, daily 100% water changes and I doubt he even needs to be in the full 10 days as what he has is most likely his natural coloration- he doesn't have rot in the damaged areas of the fins where rot forms at. So I'd just give him 3-5 days in it if you feel you need to use it.
Originally Posted by EvilVOG
Salt will kill your live plants, so either move him or the plants. There was a convo on the home page where someone recommended just water changes without salt, as using salt has become a hot button issue around here.
Those conversations were talking about a ripped/chewed fins- not fin rot. Myself and many of the experienced members here believe salt isn't needed, and many people have cured fin rot just on water changes alone- since rot tends to need the right conditions to live and with clean water changed daily, it doesn't get it. Please it's not a hot button, it's an opinion of many experienced owners and vet. Why I would suggest lessening the use of AQ salt in this situation as it's not truly needed- and exposing your fish to something that has potential to harm the kidney/liver when not needed isn't a good idea.
But.. that's just my opinion and experience.