Thanks Russell - I'm not so much stressed about it, as just frustrated *lol* it seemed they were doing so well for the last 2 weeks... but then tonight it just changed... *sigh* In any case... I have been keeping a record of any changes I've made to their tanks [you're welcome to move this thread to journals - if it's better suited there]. I do see your point though about increasing the plants and providing more darkened areas for hiding.Sorry if this comes across as harsh but ... There. Is. No. Way. To. Stop. Fin. Biting. Period. Betta owners either learn to live with it or rehome the biters. It breaks my heart to see so many people stress because think they have it figured out only to have the biting resume.
It's not your fault. It's a behavior you can't control and, after reading about it for so many years, it seems a high percentage of Betta in open and/or shared barracks have the problem. As Hallyx says, a tank isn't properly planted if one "can glance over and easily see a Betta." Then add that Betta in a divided tank share more than water; they share stress, too. And phermones.
As Betta are not bright light/lightly planted fish, you could try muting the light and adding a jungle of plants but you'd still have the barrack's problem. Perhaps experiment with one of the biters in a single, muted-light, jungle-like tank.
Knock wood, since I've started keeping tanks like these fin biting has stopped. When I had a divided tank it was this planted in both sections. Top and bottom photos are same tank but bottom photo is when the 20 long was home to Dexter, an OHM. Notice how many darkened areas there are?
When I moved the boys to solo tanks - I didn't put in their decor - well I kept the plants, added more and removed the ornaments & had different substrate; that being said, I'm willing to try it again... perhaps it was due to the plants I used (since I only had plastic - panty hose tested) - but now they have silk plants - I might have to add larger silk plants.It's not going to stop. Can't say it many more ways or plainer than that.
When you moved the boys to solo tanks did you move them with their current decor? I've observed in a large percentage of fin biters: High light + low planting = fin biting. Your tanks would be considered sparsely planted and brightly lit; two major stressors. Which is why I suggested you try a heavily-planted tank; not just replicating their current habitat in a solo aquarium.
I think this would be better as a journal. I'll do that now. Maybe someone will change from sparsely planted and brightly-lit tanks to a more natural habitat. Might bring the same results I experienced.