Wildly innacurate API liquid test result w/dry ferts?

I had spent months trying and failing to dose enough liquid ferilizer to really raise the measureable level of NO3....
Recently I switched from liquid products to a dry fertilizer mix by Green Leaf Aquariums. My API liquid test result for NO3 went from ~10ppm to 80ppm after a single dose which was measured to yield only 1ppm of NO3. A simple cheap TDS meter shows <5ppm change in solids!

I knew these tests were sort of a guess but wow didn't realize it could be off by a factor of 5x, 10x, maybe more. The TDS measurement is the final nail in the coffin for me: this test is wildly inaccurate!

The source of NO3 seems to affect the test result. In my case recently dissolved KNO3 salt shows up >10X better than whatever is in the liquid ferts I've tried from various brands.

The test is fresh, unexpired, recently purchased. My technique is correct. I did a large water change and repeated the process to make sure that I wasn't mistaken: same result.

 

Ok yes the test kit is terrible. But how did you test? Vial next to the chart or vial over the chart? Next is the green leaf ferts accurate?

 

Ok yes the test kit is terrible. But how did you test? Vial next to the chart or vial over the chart? Next is the green leaf ferts accurate?

I always put the vial in front of a separate piece of white paper to eliminate shadows.

32.6 grams measured KNO3 salt went into my 500ml solution along with other macro salts according to this PPS recipe here: Planted Aquarium Fertilizer Guide, Fertilizer Dosing Calculator
I measured the KNO3 with a digital scale with .00 gram precision. It was a little less than 2 Tbsp to get to the recipe's ~32g. This would be hard to mess up, I have used the scale for baking regularly, no confusion there.

I then added 5ml (i.e. 1% of 32.6g KNO3) of the fully dissolved solution to my 125 gallon system, which should actually yield less than 1ppm NO3.

Again, the TDS meter confirms the correct change in TDS when I add a dose of the macro solution. Contrary to the wild swing I see on the liquid test.

To get the concentrations reported by the liquid test would require vastly overdosing either the dry or liquid mix by 5x, 10x, or more. It just doesn't make sense.

 

Thing with the liquid test is your looking at colors that are so close all your doing is guessing. I put the vial right on top of the chart. If you can see the color through the vial that ain’t it. It’s the last one you can’t see
this is 20ppm.

 

Thing with the liquid test is your looking at colors that are so close all your doing is guessing. I put the vial right on top of the chart. If you can see the color through the vial that ain’t it. It’s the last one you can’t seeView attachment 867857this is 20ppm.

Thanks, This is helpful! I don't have any experience reading more than 5ppm. I think this hue is harder for my eye to judge than the ammonia and nitrite colors.

 

When it gets dark diluting the sample 50/50 with clean water can make it easier to read.