frayed betta fins?

[mybettanorman]

hi,
my betta (norman) has been in his home for three momths now. he's in a 3.5 gallon tank with a weak filter, silk decorations, 7.5 watt heater, and i change his tank water once a week, sometimes more. for the past month or so, i have been noticing a fray in this fins and it looks like someone took scissors and started cutting them into strips, and ocasionally, theres a huge chunk missing.

at first i thought it was fin rot, but there was no black or white coloration, but i took some aquarium salt and changed his tank every two days for two weeks.
it went away, and i thought it was resolved.

then about two weeks later, i was supposed to go to the beach for two days and that morning i noticed the same thing! i panicked but decided to leave it alone until i came back. i come back and it's completely healed!
this happens very frequently and it just reheals randomly, but it makes me worried. is he biting his own tail? i never find any tail chunks, so i dont know. any help? thanks!

 

[drm315]

Do you have a test kit that can test your water parameters (ammonia, nitrite, nitrate)? Three months should have produced a cycled tank. Did you cycle the tank fishless or with Norman in the tank while it cycled? My first thought is to test your water parameters and see if something is elevated that could be causing his reaction.