I do partial water change in one of my 10g tanks at least twice a week because I have two little guys that haven't quite got the hang of competing for food so I over feed a bit. When I do pwc's i treat the water that is within a degree or two of what the tank is set at. I change about 50% or so because with a 10g the gravel vacuum sucks water out pretty fast. The last time or two after doing the partial water change 3 of the 4 fish in the tank go to the top right hand corner and stay there for awhile. This baffles me... Any idea why?