05-20-2011, 11:48 AM
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Often a sudden change in behaviour is a signal of stress from something. I have no direct experience with kuhli loaches, but my other loaches definitely do this when they are stressed. I recently moved my Botia kubotai to a new tank, and for more than a week all five of them swam in a clockwise circle against the glass in the rear corner of the tank all day long, only settling down on the substrate at night. Finally they stopped, and since have been normal. They clearly did not like the move. My dwarf chain loaches and dwarf banded loaches in another aquarium did much the same when they were moved.
The issue now is finding the cause of the stress. Stress can occur from many sources: water conditions, a significant change in water parameters (hardness, pH, temperature, nitrates), other fish in the aquarium, too few loaches in the group (should be at least 5 kuhli), chemicals/treatments/toxins in the water, etc. Consider each of these to see if you can track it down. As i said, i have no direct experience with this species, so it may also be that it is "natural," though I would doubt it, but if someone with experience says it is, fine.