Some fish are more sensitive then others. A bubble nest doesn't necessarily mean much other than he was ready to breed. There are even times where fluctuations in water temp stimulate them to think it's time to breed.I neglected all of them except he's the only one who's showing illness. The others seem fine. One even made a bubble nest. And cause I changed the water before I filled that out. And sorry, I have anger issues. Where do I get the stuff to treat it tho? And if it's nitrate poisoning then how do I treat that?