Sick glass catfish?

Could I have some help here? My glass cats don’t seem to be doing too well; they’re spending some time just floating feebly near the surface, barely moving their fins. My one little cat will swim frantically at times, then crunch himself up in a corner of my tank, and keep on swimming! It’s like he’s blind or unable to feel anything at all. His whiskers are askew and they just all seem kind of listless at times…

I have a 30 gallon tank and 3 glass catfish. I checked my parameters, and nothing was too high! Nitrite is at 0.5 ppm, Nitrate around 40 ppm (I’m doing a water change tomorrow) and everything else about right. I’m a little low on Alkalinity, but I’m gonna fix that tomorrow too after the partial water change.

Any tips for how to help my glass cats? I’ve had them for a year now, and they’ve lived through everything! What can I do to make sure they stay happy and healthy?