I've been posting the very same experience, but w/ more fish. In early Feb., I had a 12-betta sorority. They started dying off, one at a time. I've lost 7 fish (5 girls & 1 male) in 3-week increments ever since. It's been heartbreaking.
They've displayed symptoms of "sulking" one pinconed, and the others, fin rot. I'm treating w/ Kanaplex, but 3 girls look really bad. AND I'm scheduled to go on vaca next week... I don't know what to do.
Oops, lost an oto cat yesterday, too...
4 bettas: Butch, 3-year-old violet male PK EE & 3 girls in a 30 gal sorority. I do not name the girls anymore! 1/2 of them die within a month.
30 gal: 9 Oto cats, 4 Ocellifer cats. Inverts: Cherry shrimp & 2 assasin snails.
10 gal.: Butch, his 5 oto cats, 3 assasin snails, and a few snails the assassins haven't caught.