Thank you for waiting. This infection is usually caused by gram negative bacteria. BettaFix and MelaFix are too mild to treat it. Maracyn 2 is most effective against this type. Other antibiotics, including tetracycline can be also used, but I've found the Maracyn 2 to be most effective. If you can't find Marcyn 2 in your pet store, look for another antibiotic that treats gram negative bacteria.
There are some other things to consider. The tank should be kept at 76*F to 78*F because those temperatures inhibit the growth of the bacteria. Water quality must be perfect. I would do a 100% water change to remove the BettaFix, using water that has had the chlorine removed and is the same temperature as what is in the tank. Remove carbon from the filter. Then treat with Maracyn 2 according to the package directions. If water conditions are not kept perfect, no medicine will work. It's best to test for ammonia and nitrites every day. You want them at zero. With perfect water, optimal temperatures and an antibiotic that treats gram negative infections, you should be able to clear up the fin rot.
If you have more questions, let me know . I hope your betta will reach a full recovery.
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