Weird results from different API nitrate test kit bottles

[B1gJ4k3]

Thursday is water change day at my house. Every time before I do a water change, I like to test my nitrate levels to see if/how much I need to fertilize. At some point along the way, I ended up using bottle number 1 of the nitrate test faster than bottle number 2 (messing up tests, spilling the vial halfway through, that sort of thing). Being the frugal (cheap) gentleman that I am, I wanted to use the second bottle up of my original kit before opening it on my new kit. It's not expired, but it's definitely older.

So, I did a water test with bottle number 1 from my new kit and bottle number 2 from my old kit. I did the test correctly like I've done hundreds of time before (10 drops from bottle 1, turn vial over a few times, shake bottle two for 30 seconds, 10 drops from bottle 2, shake for a minute, wait five minutes, view results). Nitrates on both tanks came out at over 80ppm plus!

Before I panicked and did a 90 percent water change on both tanks, I used some Coop test strips to double check. It's so hard to tell what the level actually is, but I could tell it wasn't anywhere near panic levels from the first test. I decided to do another test with both bottles from the new kit. Same procedure. This time, my one tank registered 10ppm and the other registered 20ppm (much more in normal ranges).

I would think that it wouldn't matter which bottle the test came from since they should be essentially the same, but maybe not? Is there something else that I might have done to cause such a crazy high nitrate reading? I did test another tank right before, but I always make sure to rinse the vial and cap thoroughly before testing another tank. Has anybody else had this experience?

[Guppysnail]

I was an api liquid die hard for many years. The slightest human error messes it up  or even forgetting to shake the bottle properly one time destroys the ratio of chemicals in the bottle therefore resulting in a thrown away bottle. It has taken me several months to come to trust coop strips but now I won’t live without them. Over the years I found actual accurate numbers to the number are less important than consistency of test and where my tanks run as well as ease of use. I did try using up the bottles vs scraping both when the first runs out. That never went well. I still have my api tests but since I’ve come to trust coop strips to give me consistency I can’t bring myself to go through the hassle of api liquid. 

[gardenman]

"shake the bottle properly" is a big one. How long is properly? Some say three minutes. Some people say five minutes. Some people say thirty seconds. I've gone to all test strips these days. They're just easier and probably cheaper overall. Are they as accurate? More accurate? Less accurate? Dang if I know. But they have far fewer variables.