strange change in water parameters
I use a 29 gallon tank as a water conditioning reservoir for changing the water in my discus' 75 gallon tank. I run 300 gph HOB filter with peat to drop the pH to prepare it for the next water change in the 75g. My tap water has a pH of about 7.8, a very low GH and KH each of 3 degrees, 10 ppm of nitrate, and 0 ppm of ammonia and nitrite.
I filled the 29g about one week ago. As usual, the pH has dropped to about 6.8 and the hardness has stayed the same. However, the nitrates have increased to 20 ppm and the nitrite test is now reading 5 ppm.
Does this make any sense? This is a clean tank with no fish or plants, no substrate, and it was emptied and cleaned a month ago for this purpose. This is the third batch of water I've used it to prep, and the previous two times it did nothing strange.