bettafix should NOT be used as medication used to treat finrot. It's kind of like a mild antiseptic and helps promote healing. I usually use a little in the treatment though to help promote fin regrowth.
Be careful though, to use the proper dose and NOT OVERDOSED. But that's also the case in any medicine... so measure carefully.
To treat more severe fin rot, I recommend using Maracyn+Maracyn-two combination. To measure the proper amount of medicine you should be giving in this case, what I do is take a tiny resealable jar (you can find one at a dollar store) and make sure it's clean. Then add 20 tsp of clean treated water and add 1 package of each Maracyn and Maracyn-two and mix until they're well combined. I then measure out 1 tsp per gallon of water the fish is being treated in and add it to that tank. Also, use plastic measuring spoons, it's safer so that you don't introduce any unhealthy metals to the water. I than add a few a bit of bettafix (reading instructions carefully. I sometimes half the dosage so that the fish isn't stressed out by all the medicine in the water). I also sometimes I add bettafix straight to the Maracyn concoction, but it could change the potency of the Maracyn.
Keep the container of medicine since you will have to re-add 1tsp per gallon every 24 hours. Recommended treatment period is 5 days, but I've read that in some cases people recommend 10 days. I suppose it depends on the severity of the case, but I suggest that after 5 days of treatment, do a 100% water change with aged treated water and don't re-add medicine. Watch him for two days and if the fin rot doesn't act up, he should be fine. In most cases, the 5 days are enough, so afterward just make sure you keep his water REALLY clean.