Nitrate test kit - The Planted Tank Forum
 
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post #1 of 7 (permalink) Old 02-20-2006, 03:15 PM Thread Starter
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Nitrate test kit

First, a little background. I started dosing EI back in November. In December I added Discus to the tank. Since the Discus experts always preach about the need to have low nitrates, I got an AP nitrate test kit. When I tested the tank water I was shocked to find that the test kit registered a number I couldn't read. It was somewhere around 80 or more.

I tested my tap water that I use for water changes and got a reading of 10. At that point I figured the heavy feeding and tap water were enough to maintain nitrates and stopped dosing KNO3. Ever since my tank has always tested out to 10. L repens wasn't really red, and I attributed that to the fact that I couldn't get nitrates lower.

I started getting fuzz algae, and then noticed some BBA. The Excel overdose treatment got rid of it, but the fuzz came back as soon as the treatment was stopped. All this time I kept cranking up the CO2 but that didn't help.

Finally, I decided to test my test kits. Everything tested correctly - PH, KH, GH, N, and P. Since I was still dosing KH2PO4 and K2SO4 at good levels, I decided to add enough KNO3 to bump it up by 5ppm, figuring that would take the tank to 15. It didn't, the test kit still read 10.

All this has me believing that something in my water is tricking the test kit to read 10 when it is something much less. I am going to slowly increase the KNO3 dosing to see what happens to the readings, but the effects are already apparent. The algae seems to be melting and the L repens is turning red after one day.

Any thoughts on what the source of the false reading could be?
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post #2 of 7 (permalink) Old 02-20-2006, 03:42 PM
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Yeah, what kind of test kit are you using? Even the Lamotte's aren't accurate to any closer than 3-5 ppm or so. Your inability to detect a difference may be no more than the sensitivity of the kit. Also, remember that levels can fluctuate from day to day and from week to week. If you get water from a city supply, they often use different lakes, streams, wells, and other sources at different times of year. Overal system demand also impacts how much 'stuff' gets into the water on its way to your house. Your exact plant mass and the timing of your trimming will affect uptake rates too.

The other possiblity is that your tank is now nitrate limitted. When you add some it is almost instantly taken up by the plants and you don't see much of a bump. I'm not at all surprised that you were in the 80 ppm range with EI. It's happened to me before. I attributed it to an overestimation of my lighting (and therefore uptake) and an underestimation of my % WC each week. I'd keep checking and keep adding NO3 until you get to the 20 ppm range.
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post #3 of 7 (permalink) Old 02-20-2006, 03:53 PM
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Try to test some water that does not contain nitrates, like bottled water. This should set your zero reading. Use that water to prepare a 10 ppm solution, and test that too. Maybe one for 20 ppm too.

It's been discussed that AP testkits show false positives when they get older/turn bad. They last only for a year or so.

I don't trust the absolute value that they show. I test my 3 tanks and tapwater, and look at the differences. That tells me if one tank is running low, or another one needs reduced dosing. The absolute value is what you get by dosing a certain amount of powder into a certain volume of water.


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post #4 of 7 (permalink) Old 02-20-2006, 04:29 PM Thread Starter
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Quote:
Originally Posted by Wasserpest
Try to test some water that does not contain nitrates, like bottled water. This should set your zero reading. Use that water to prepare a 10 ppm solution, and test that too. Maybe one for 20 ppm too.

That is exactly what I did. Distilled water tested 0. I added enough KNO3 to take it to 5 and that is what the test read. I added more to take it to 10 and that is what the kit read. Did one for 20 and that was correct.

My tap water is well water. I have tested it many times and it always reads 10. The test kit is 2 months old. (Assuming F&S didn't sit on it for a year.) Can the AP kit read something else as nitrates?
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post #5 of 7 (permalink) Old 02-20-2006, 05:25 PM
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Hmmm... that proves that the test kit works. If it shows zero with distilled water and 10 with tap water I would assume that would be correct then.

So the question is why adding KNO3 to your tank doesn't reflect on the test.

Couple of ideas...

Did you add the correct amount for your tank? Was it KNO3? Was it mixed well within the tank water? How long after adding did you take a sample (did the plants take up the added NO3 already?).

Just to verify one more thing... fill a 5 gal bucket with tank water, take a sample, then add enough to add 5 ppm to the bucket, stir for a while, take another sample, and measure both. See what you get there.

IIRC the presence of Nitrites can skew the test results, but your Discus would probably complained by now.


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post #6 of 7 (permalink) Old 02-20-2006, 07:00 PM
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AP test kits can be inaccurate, and can change over time. To get a more accurate reading, test both your calibration solution and your tank water in two different tubes at the same time. Set the calibration solution to the level you want your tank water to be. I've found that the AP test kits are very inaccurate between 10 and 20. When the nitrates are only slightly above 10, you will read 20. When they are 20, you'll still read 20.

Same goes for between 5 and 10. Between 7 and 15, the color can look the same (or not- each test kit I use seems to be at least slightly different). However, if you compare it with your calibration solution, you can tell the difference.

So, to be accurate, test your tank water at the same time as your calibration solutions, and compare with your calibration solutions, not the color on the charts.
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post #7 of 7 (permalink) Old 02-21-2006, 02:18 PM Thread Starter
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Quote:
Originally Posted by Alight
AP test kits can be inaccurate, and can change over time. To get a more accurate reading, test both your calibration solution and your tank water in two different tubes at the same time. Set the calibration solution to the level you want your tank water to be. I've found that the AP test kits are very inaccurate between 10 and 20. When the nitrates are only slightly above 10, you will read 20. When they are 20, you'll still read 20.

Same goes for between 5 and 10. Between 7 and 15, the color can look the same (or not- each test kit I use seems to be at least slightly different). However, if you compare it with your calibration solution, you can tell the difference.

So, to be accurate, test your tank water at the same time as your calibration solutions, and compare with your calibration solutions, not the color on the charts.
Thanks, that is very useful info and would explain my problem. The tap & tank water probably have just enough nitrate to register 10, and trying to bump it up a little didn't change the reading. I added some more KNO3 and it did test above 10. Now I just need to keep from dosing too much.

Great idea about comparing the tank to the calibration solution. I now have about 495ml of it, so I think I will be able to test any kit I buy in my lifetime!
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