+1 on getting more otos. That should definitely be your first addition. As we all know from stories of bettas and goldfish living for years in dirty, small tanks, just because a fish is alive doesn't mean that he has has a good quality of life. Especially in a tank with a semi-agressive fish like a betta, otos need a group for security. The absence of other otos indicates to a shoaling fish that they have skedaddled because it isn't safe, and the poor lone oto will constantly live in an environment where he feels he should leave for survival purposes but can't.