Keep the one treatment going. After a while the fins can be reduced to stumps!
Exposed fin rays, blood on edges of fins, reddened areas at base of fins, skin ulcers with gray or red margins, cloudy eyes. These may all be present. The worse it gets the more likely another type of infectious agent may take advantage of his condition.
Most breeders concur it to be a bacterial infection of the tail and/or fins and usually caused by generally poor conditions or a bully tank-mate. If aquarium conditions are not good an infection can be caused from a simple injury to the fins/tail. The tail and/or fins become frayed or lose color. Over time the affected area slowly breaks down.
Once you first figure out what is the cause, you remove it. If it is poor water conditions (the main reason) then fix it, clean their bowl and continue to keep it clean. (don't forget a good reading can hide a spike if you only check once or twice a week)
Also, treat the water or fish with antibiotics. feeding is more effective but the common packaged brands are hard to find (prolly because it HAS a shelf life posted..unlike regular foods which go by lot#)
When the first traces of finrot appear, The most common practice and suggested one is clean, clean clean. Clean their water daily treating with salt. This usually will take care of it. If that doesn't work : Neosulfex, Jungle Fungus Eliminator, Amipicillex, Mela-Fix(the betta safe type-bettafix. Melafix is hard to measure out in safe doses; breeders who use it mix a whole 10 gallon batch and then use it), Tetracycline
You can prevent Tail/Fin Rot by keeping them clean. Also, do NOT use same net/container for healthy fish. It is considered very contagious.Ii have not had an issue with it only because all our Betta fish are in divided 10 gallons except 4. The only animal that had it was my poor departed Christmasfish (who had New ich, columnaris, and rot all at once....and lice). And the 10 gallons allow for more stable water control and better filtration when it is 3-4 fish.
In your case the cause is removed...keep it pristine clean don't wait for readings...put him in a smaller container if needed to do a frequent(every 2-3 days) water change out and be sure he is at least in his mid range (stable F 78-80) of temperature to strengthen his immune system. Bettas can live at cooler temps but they are still tropicals and get weak at lower temp ranges.
"Fish! Thank You! Oh, tropical huh? Did you buy a heater? Auuugh!"
Mega-pet stores prolly should not be allowed to sell animals as retail "items".