Aside from clean water, a little aquarium salt will help! Use 1 teaspoon per gallon. Try not to use the salt for more than 10 days as it can hurt their kidneys. Also, Stress Coat + could help him quite a bit. It's a special water conditioner that has aloe in it to promote fin regrowth =) He's absolutely stunning, he has really nice finnage!!
That would depend on a lot of things (the strain he has, level of care, health of the fish, etc), but since it hasn't changed too much since you've had him then he'll probably be ok. The treatment for finrot is fairly easy. You'll have to quarantine him for a few days. The cup you got him in will work fine. Is your tank heated? If it is, then float the cup in the tank to maintain proper temperature (if it won't float, you can tape it or clip it to the side with any type of clip). In a gallon jug (milk jugs work great, just make sure you rinse it first), pre-mix a solution of aquarium salt. 1 teaspoon per gallon should be enough. Also mix in the Stress Coat + if you can get that. Use that pre-treated water to change his water everyday until he gets better. Only do the salt treatment for around ten days. I doubt he'll need that long as it looks like he has a very mild case. Another thing you could add to speed up the process would be tannins from either naturally dried and crushed oak leaves or DECAF green tea (just green tea, no lemon/mint/other flavorings). Steep the leaves or the tea in the gallon jug and let the water get really dark. But these are optional, the salt is the most important thing.
alright, thank you so much. does he still have to be quarantined even if he's in a solitary tank? i can't get some aquarium salt or stress coat right away, probably in a couple days. is that an okay length of time to wait for treatment?
The quarantining is just to make the daily 100% water changes easier on you. You don't have to QT him if you don't want to but it does make things tons easier lol! He should be fine for a couple days without it, just keep his water really clean and watch him really closely for any changes =) I hope his tail heals back to its full glory, he really is magnificent!!!