Erratic Nitrate readings

I have a planted tank that I've been testing with API Master Test Kit for the past three weeks.
I get it that plants consume Nitrate but after the disparity between today and yesterday's results I'm doubting the accuracy of my Nitrate test.
I have a log of the past three weeks of testing results here: Ebreus' Cycling Thread - In progress. Updates: 12 | Aquarium Nitrogen Cycle 433762
For the sake of not having to open another tab if more information really isn't necessary here's the last several Nitrate readings from oldest to most recent:
80ppm, 80ppm, 80ppm, 160ppm, 80-160ppm, 20ppm
Starting with the last 80ppm reading I changed my ammonia source from fish food to 10% Ammonia so I was expecting a change there but the last change doesn't make much sense to me. I am as careful as I can be to make sure the test is done properly and everything is shaked for as long as it needs to be.

What do you think? Is this just a bad reading or could something have really reduced my Nitrates to 20ppm without a water change?

 

I have a planted tank that I've been testing with API Master Test Kit for the past three weeks.
I get it that plants consume Nitrate but after the disparity between today and yesterday's results I'm doubting the accuracy of my Nitrate test.
I have a log of the past three weeks of testing results here: Ebreus' Cycling Thread - In progress. Updates: 12 | Aquarium Nitrogen Cycle 433762
For the sake of not having to open another tab if more information really isn't necessary here's the last several Nitrate readings from oldest to most recent:
80ppm, 80ppm, 80ppm, 160ppm, 80-160ppm, 20ppm
Starting with the last 80ppm reading I changed my ammonia source from fish food to 10% Ammonia so I was expecting a change there but the last change doesn't make much sense to me. I am as careful as I can be to make sure the test is done properly and everything is shaked for as long as it needs to be.

What do you think? Is this just a bad reading or could something have really reduced my Nitrates to 20ppm without a water change?

Are you doing the nitrate test according to the instructions? In other words, shaking the bottle 2 for 30 seconds and then shaking the test tube for a minute?

 

In some circumstances I have seen the nitrate test default down to 5 but never 20. This has been known to happen when both nitrites and nitrates are off the charts. Have you done any water changes since you started this cycle? If not it is probably time to do one.

Your day 21 reading leads me to believe that this cycle is done.

Day21: pH: 6.8, Ammonia 0, Nitrites: 0, Nitrates: 20 - Added 3.25ml 10% Ammonia

 

I would think the 20 reading was false. It's more likely a mistake was made once than 6 times. If plants could consume nitrates at that rate i'd buy more plants. Why didn't you retest after that weird reading?

 

In some circumstances I have seen the nitrate test default down to 5 but never 20. This has been known to happen when both nitrites and nitrates are off the charts. Have you done any water changes since you started this cycle? If not it is probably time to do one.

Your day 21 reading leads me to believe that this cycle is done.

Day21: pH: 6.8, Ammonia 0, Nitrites: 0, Nitrates: 20 - Added 3.25ml 10% Ammonia

Oh, thanks for pointing that our I had erred in my input on the log. The readings for day 21 was:
pH: 6.8, Ammonia 0, Nitrites: 2, Nitrates: 20
Sorry about that error. I haven't done any water changes since the start. I could start changing the water if you think that'll help or was that contingent on my cycle likely being complete?

 

I would think the 20 reading was false. It's more likely a mistake was made once than 6 times. If plants could consume nitrates at that rate i'd buy more plants. Why didn't you retest after that weird reading?

Sometimes I can be a bit dense. It just hadn't occurred to me to test again.

 

Oh, thanks for pointing that our I had erred in my input on the log. The readings for day 21 was:
pH: 6.8, Ammonia 0, Nitrites: 2, Nitrates: 20
Sorry about that error. I haven't done any water changes since the start. I could start changing the water if you think that'll help or was that contingent on my cycle likely being complete?

Since you've not done one I highly recommend you change out no less than 50% of the water now. A water change is never a bad idea when something like this happens. Cycling a tank uses up the minerals present in our tap water. After a while most of it is depleted. I know a lot of places don't recommend water changes during fishless cycling but I can't count the number of folks that come here having the same problems you are having. Most of the time a water change will get everything moving in the right direction again. Often folks find that their cycle is actually done after the water change.

 

Okay just tested again today and got 20ppm Nitrates... then redid the test and got the same result.
I remember hearing somewhere that if Nitrates is off-the-chart high it'll read as 10-20 range and given I got top of chart readings before this I'm inclined to believe that's what's happening here.

 

Are you doing the nitrate test according to the instructions? In other words, shaking the **** out of bottle 2 for 30 seconds and then shaking the **** out of the test tube for a minute?

Yes, I shake the bottle and tube as vigerously as I can for the listed duration and regularly add 5 seconds to be sure it's shaken well enough.