Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Test your tap water every time. Tap water can have nitrate, especially if you are in an agricultural area, but even city water can have NO3 occasionally.
Try doing a water change this way:
Set up a bucket of water with an air tube in it (I rubber band a rock to the air tube to keep it down) higher than the tank.
Tie a loose knot in the air tubing.
Start a suction in the tubing, then snug up a bit on the knot. You want a steady stream, but not too much. (You might be able to handle the full flow without the knot)
Clip the tubing so the flow is horizontal, just under the water level in the tank.
Now start another air tube flowing out of the tank into a bucket on the floor. Keep the intake of this one fairly high up.
Goal: VERY slow out, matching in, and neither gets anywhere near the substrate. The water is not churned up at all in any way that could affect the substrate. Monitor the flow, and make sure the water level does not drop too much.
It may take an hour to do a water change, but then you will know that there was absolutely no way of disturbing the substrate.
Then monitor the NO3 through the week.
The small dose in that large a tank (1/8 tsp or less in 29 gallons) should not have raised the reading by enough to show as a color change between 2 colors on the test.