Nitrate weirdness

[LabRat]

So off the bat, this might all be because my test strips are unreliable. I got them because they were on sale, and I figured having something was better than nothing. I plan to get a better test kit when time permits.
That said...
My Nitrate levels have been all over the place for the past week. On Sunday before I changed the water, ~40ppm. After a water change (50%) it shot up to ~60-70ppm, and then dropped back to ~40ppm 3 hours later. I performed both of the post water-change tests twice because the results seemed weird.
Since Sunday, I've tested at least once daily and I get results ranging from 20-50ppm, and now I just got a result of 10ppm. Repeated that test 3 times, consistent result.

I haven't done any water changes since Sunday, and the fish are very active and eating well, so I'm more confused than worried at this point.

Any ideas what could be causing these fluctuations? Should I just throw out the remaining test strips because they're worthless?

[wolfiewill]

Your 'test strips' can't be trusted. I use Seachem Nitrite/Nitrate test kits a lot and am very pleased with the repeatability of the  results.

[exv152]

I personally don't rely on test strips that you dip in water, IME they're the least accurate. Check the expiry date on the kit too. If you're really concerned about your nitrate levels, you could cross reference it with another reliable test kit like Seachem, red sea, salifert or nutrafin. I personally don't like API, I find the colour samples too similar after 20ppm.