If the fish stay around only one side of the tank, it means something is wrong with the other. When I had an air bubbler in one side of my twenty gallon, all the fish stayed over on the other. I suspected the bubbles, so I took it out, and sure enough in less than an hour the fish were back to utilizing the entire tank.
I have also noticed my fish tend to gravatate towards the light. When my tank hood lamp is off, the light primarily comes through the side by the window, so the fish seem to stick around the light side more than the darker one. When I turn the hood lamp on, again they utilize the whole tank space.
I've also trained them to feed from one spot, as Cackett has done with his fish. At feeding time they all congregate in that spot, or even if I sit in front of the tank, they go to that spot as a conditioned response to someone before the tank.
Where does your filter come out, is it too strong? Are there bubbles on the side they are avoiding? Is the temp different (get two thermometers and put one on each side of the tank to measure temp at the same time to see if there is a gradient). In order to find out why they have a preferance you must remove as many variables between the two sides of the tank as much as possible.
~~Colesea
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