plants use O2 at night. which could deplete the O2 for the fish during night hours. but since you dont have live plants you dont have to worry about that. do you test your water? what are your params. its natural for plecos to gulp air, but they shouldnt be doing it to often. fish that are able to use surface air are unable to fully survive with just gill extruded O2 and must supliment w/ a gulp of surface air now and then.