Betta Fish: Help. Frayed tail.

Today I woke up and came across this! It's been a week since my other betta had these symptoms. I bought Aqualife to use, and I put it on this one too to prevent it. Could it be a reaction? I did the tests and only the pH seems high to me. Temperature 25°- 26°C. I do TPA every week. Help me, what do you do to treat it?