Never seen this before... "clamped" betta fins

[pnwbettas]

This has been going around my fishroom and I am at a total loss. 5 bettas infected now. All cycled tanks, planted, regular water changes, but some of my betta are having major fin issues. Their fins become smooshed looking, almost clamped. They cant extend them all the way when they flare anymore. I have never seen or heard of anything like this. If you have ever read anything about this or had experience with it, please let me know. I've been trying different antibiotics assuming it was bacterial but nothing is working.

Ive already tested my water a thousand times, done aquarium salt, all tanks I keep with tannins. I am stumped