I'm trying to figure how you would end up with such high NO3 readings doing 50% water changes weekly. I guess I've always assumed that with this water change regiment you would have to dose everything at some level. Is you tank really got that high of a bio-load that it can build NO3 that high with 50% water changes weekly? Is filtration an issue?
I just got a Seachem kit to compare it's results with the API kit and they were fairly close. The Seachem looked like it was maybe 5ppm less than the API. Man, it's hard to get an accurate color comparison. You could easily be off that much depending on how you interpret the color.
Just thinking out loud here, if your reading was close to say 40ppm couldn't you mix your aquarium water with an equal amount of RO water, fill the test vial with that and expect to get a reading 1/2 the actual aquarium NO3. If nothing else it might reduce the test level down to something easier to read on the color chart.
Probably some background info would help. I do not run full EI dosing and water changes. When first getting serious about the testing as I restarted plants, this area was in a severe drought. Water restrictions were in place and the local water supply lakes were down to 30% and dropping.
I was also running about a dozen tanks and breeding fish. The combo made it almost criminal to consider dumping that much water weekly and I also did not want to plant fully. That was never my intent as I use plants to add to the tank appearance while the fish are the main interest. With breeding going well and less planting than normal, full EI was not the best way for me to go. EI does eliminate the testing but as I became interested in all the talk of testing and I did have access to kits, I tried testing and got no usable results when using the API liquid tests. In building samples for testing, I used both API and the Jungle brand test strips and found the strips came out close on 25 PPM samples, while the API was just red or too red with no usable readings.
My current situation has changed as I now have few tanks and the only one which I look at much is only a 120. The drought situation has also cleared for the time being so that dropping 60 gallons is not the critical question it once was when we were looking at running out of water. But the question of testing is still a bug!
So I did three 50% water changes over the last week and tested each time with no definite color change. It is hard to say for sure that the test today is the same as two days ago but I can detect no change by looking at the chart.
With African cichlids who are supposed to require good water and still continue breeding well, and tests that say I have more like 80PPM, I have to feel the test is not doing me much good.
This is an old issue for me and just comes around again. This is an old posting about the same question on nitrate: