Should I worry?

[Tarabull]

I rescued Ayudame a few weeks ago. He (possibly she) has put on weight enough to not show his hip bones & his gills have started to get better too. His tail is still really thin. The problem is he still hangs out at the top of the tank on the filter or on decor. It isn't involuntary because he has no problem staying on the bottom. The water tests are good & the other two don't act that way. So is it just a quirk that he enjoys? Is it because of his small gills & he is trying to get air? Or is it something else?

 

[Kaysie]

Oxygen saturation is highest near the surface of the water. It's likely that he's just lazy; instead of swimming up for air, he just lays around at the surface.