If it had fin rot already you should of not brought it or return it to be honest.
You can treat with a salt bath.
get a litre of water add 30g of salt mix it well.
add the fish.
Fish dont like this, the float and strougle.
leave then for 60 seconds at the most and return to tank.
treating with melifix or myxazin are the best stuff though, just add every 2-3 days.