I just finished adding a new live plant and some more ghost shrimp to my 10 gal tank, when I noticed Mo (white), Larry (orange), and Curly (yellow/black), my fairly new koi, gulping just bellow the water surface. these guys are only about 2 inches long, and I haven't seen them doing this before for the 2 weeks I've had them so far. I checked my pump and filters, no clogs, so I doubt there's been any significant change in aeration. I know a 10 gal tank will be too small eventually, especially with a 2 inch pleco (Shemp) and half a dozen ghost shrimp, but so far there hasn't been any obvious problems with the oxygen level. testing ph, ammonia and nitrite all look OK too, though I plan on doing a 30% water change tomorrow (I want to give the shrimp at least a day to acclimate).They've been sporadically rising to just bellow the surface for a while now, and I'm starting to worry. I cant help but think they're not getting enough oxygen, but they're not actually gulping air from the surface, they're mouths are a good couple of millimeters below the water line.The other thing I've noticed is that when they're not flirting with the surface, they're congregating to the back corner of my tank closest to the intake and water-fall of my filter pump. I don't know if they're taking advantage of it as a bubbler, or just enjoying the flow, but if my tank does have an oxygen problem, it seems to me to be something I should fix rather urgently.any suggestions?