Kiarra
Former Betta Breeder
I do have a suspicion.
It sounds like when he's separated, he does well. And when he's around other Bettas, he doesn't do well. Maybe it's stress at being around the other Bettas? I really think his tail looks like he's been chewing on it. I've seen that same kind of tears in the boys I bought from Petsmart that were in the same cup and had torn into each other. Those look like bite-rip marks.
I think you might want to look into letting Sakana live all by his lonesome, away from the other Bettas, for a while, and see if that fixes his symptoms.
It sounds like when he's separated, he does well. And when he's around other Bettas, he doesn't do well. Maybe it's stress at being around the other Bettas? I really think his tail looks like he's been chewing on it. I've seen that same kind of tears in the boys I bought from Petsmart that were in the same cup and had torn into each other. Those look like bite-rip marks.
I think you might want to look into letting Sakana live all by his lonesome, away from the other Bettas, for a while, and see if that fixes his symptoms.