I suspect the AP test kits SMG and I (among many others; very common kits) use are reasonably correct. I tested my main pond this weekend, just previous to a 30% water change, and found my nitrates at ~9ppm.
However, the kit's color card only has figures for 0, 5, 10, etc. So guesswork is involved for in-between colors.
My 9ppm was just a little lighter than the 10ppm color. The 7ppm color is in-between 5 and 10.
But taking any of these as accurate to anything better than +/- 1ppm isn't advised. :roll:
EDIT: The *stuff* associated with spawnings in my pond is much less than with more mature, larger, fish. My older fish are only 3-4 years old and the older ones aren't from jumbo stock so only a few are over 18 inches in length. All are less than 24 inches.
And, of course, after the 30% water change they spawned yet again. But the filtration, including significant shower type with a little bit (20Kg) of BH media, seems to be keeping things very healthy. Spawnings don't seem to effect water quality much, if at all. Excepting the visual aspect with all the foam, of course. :lol: