My 4 male guppies usually swim at all levels in the tank but since my last water change they have started swimming around the bottom only. They don't look ill and swim around the bottom quickly. I have just tested my water with a liquid test kit. Ammonia is 0, Nitrite is 0, nitrate is 2.5, pH is 6.90, KH is 3, and GH is 5. All these are more or less as they have been for months. The tank is 12 Gallons, well planted, has sponge and ceramic tube filter with a cascade overflow and a air stone as well. The temp. Is a steady 25.5. Can anyone suggest a reason for this change of behavior? Is this a sign of a problem to come?