Livebearers have been inbred so badly, they are now what I call "sensitive" fish. As such they'd be the first to show signs of trouble. Ottos are also sensitive to water quality and the inevitable disease outbreaks leading from that. Your best bet is now to get serious about water changes and reset the tank. If it were me, I'd sacrifice a bit of time and start 60-70% water changes every other day for two weeks or so. Even if the water is not the issue, it can halt whatever is going on in there. There's nothing like fresh water when it comes to fish health. Even cleaning the substrate or filters isn't that important. Just drain and fill....