Horrible fin rot

My betta has horrible fin rot that’s been reoccurring for months. I’ve tried everything from tannins, salt baths, more regular water changes, even methaline blue to try to treat him. His fins seem to start to grow back, and then it gets horrible again. Besides the fin rot, he is happy and healthy. Tank has nothing but live plants, rocks and wood for tannins. Occasionally I add Asian almond leaves.

His tank is well cycled, five gallons, water perimeters are fine. I feed him frozen blood worms along with pebbles and the occasional freeze dried brine shrimp.

I’ve attached photos of him when I first got him and him now.