tail rot?

Hi! I've had a betta fish for a few months now, and with every water change his fin recedes and has long tears along it. Initially I was told it was most likely fin rot, and I tried using bettafix and trisulfur tablets which helped a little, but would never cure his tail leaving it as bad as it...

i just got this betta a few days ago right now the betta started to lose a bit of its tail it used to be like this now its like this does anyone know whats wrong with this betta? it will be appreciated

Tank size: 6 gallon (Fluval Edge) pH: 7.5 ammonia: 0 nitrite & nitrate: Unable to measure myself, had it tested at the fish store this evening and they said the levels were low and fine. kH & gH: Unsure. Water is city water. Always treated with appropriate ratio of Nutrafin African cichlid...