Ok, now I've heard from at least three different people and some books: 'if something is out of balance, stop feeding'. It's very hard for us to imagine but fish can apparently stay happy for weeks without food. Is this an idea?
Another question: what's the role of oxygen in this story? If CO2 is high, is oxygen low? Or doesn't it have a relation? Because you said CO2 makes it difficult for fish to take up oxygen, didn't you?
And another one: if CO2 would not be the cause of this situation, does low pH and high KH still automatically mean CO2 is high in the water?