I'm fairly new at this, about 3/4 year, and like you it would seem to be a water issue being as the problem arose when the hose was left on. Do you have an aireation system in the tank? If not, that would be a cheap addition to the system to eliminate the lack of oxygen issue. Have you stopped feeding for a while? When fish are stressed that should be done as you attempt to solve the problem. They can go quite a while without food. I'm sure some of the more knowledgable people will chime in, but this is what I would do first.