Betta Fish: How to get rid of stress stripe?

Hi guys!

This is kiko who i got about a week ago and is my first fish. I noticed when i got him he might have some fin rot on the upper and lower fins and he came with a stress stripe down his body.

I very stupidly cracked his tank yesterday trying to move it incorrectly. Because of that, i had to remove him and make a new tank + fish in cycling (ideally i would never do). Kiko remained unharmed, but since he got put into the new tank overnight his stripe got way worse. On the bright side, hes still a super greedy eater and active! Im monitoring the water and changing it everyday. Any tips on how to reduce his stripe? Would adding tannins help? I also was wondering if it would be better to wait to treat his fin rot til his stress goes down.