api makes an anti fungal that zaps fin rot. it is a 4 day treatment w/o carbon in the filter then add carbon to filter and bingo done. it turns the water green so don't worry about that unless your silicone sealent is prone to discoloration. also it will cause acrilic to pick up a green haze.
i also used it on my wifes black moor who a few days after purchase developed a white fungus spot and it zaped that too. *it was fungus not ich*
the betta i used it on had a fairly quick moving advanced case of fin rot and now it is doing good.
*good is defined as; active, alert, no more fins rotting away, fins regrowing a bit, having deep rich colors.*
after the fin rot happened we moved him to a 10 gallon tank and put in some "betta bulb" plant unknown what they are but they are growing out like a grass or twistedy water onion. and he likes to swim into them and then come out and say hi.