I got my male betta about 2 months ago. He was doing really well until now. The past 2 days he's been hiding and sitting on the bottom of the tank, and looks like he has trouble breathing. I haven't noticed any visible markings on his body, except for a little bulge posteriorly on the left, but it might be insignificant (hes had it the entire time). I was wondering if anyone had any idea of what might be wrong with him?! The only thing that's changed in the past few days, is that I changed the filter sponges (not all). But I also changed some of the water, so I don't think its a nitrate problem.