false positive for NO3

[sillygoose]

Maybe I'm hoping against hope, but are there thinhgs that can cause a false high reading for nitrate? I ask because I've tried three different kits - one brand new - and get ridiculous readings (160-ish). So it's not a matter of kits. Some of my corals don't look great, but I still have SPS growth. How can that be? Any thoughts? I was wondering if there is some kind of matrix interference that could be giving me goofy results.

 

[Ron Reefman]

Have you tried mixing a small amount of new saltwater and trying your test kits? If the numbers are zero or very low, try adding some DT water to the new water and see how much it goes up. A 50/50 blend of water should give you a reading of about half (80ish?). I've never heard of anything causing false high numbers. Did the high numbers creep up over time or did it happen all at once? Or have you not tested your water parameters in too long a time? Have you lost a fish recently? how long has it been since you have done a water change? What % change did you do? Did you test then?

 

[Randy Holmes-Farley]

Nitrite will give a false positive for nitrate with some kits, but if the tank isn't cycling and you do not have a new denitrator, nitrite is unlikely to be present in sufficient quantities to be an issue.

 

[sillygoose]

Thanks for replies.

I did test for NO2 and it was zero so that's not it.

This did seem to jump up pretty quickly. I haven't lost any fish lately but my chaeto seems unwell - not rotting or anything - just not robust.

My skimmer has not been making much skimmate so I've been fooling with it. We cleaned out its pump this weekend so we'll see if that helps that issue.

I go for periods where I don't test if everything is going well but I've had some corals showing some STN lately. Last week I took my water to my LFS to have them test it because I thought I'd see what they got (turns out my technique is much more anal and careful). Anyway, on multiple analyses they got results ranging from 0 - 160 depending on the test kit they used. Both were API tests. I left feeling like that was an exercise in futility. My home test showed about 40 mg/L so go figure. Now I'm looking at way more then I can read colorimetric-wise - like 160. That's why I wondered if something else could be affecting my results.

 

[bertoni]

Other than nitrite and the general issue with test kit reliability, I can't think of anything obvious. I might try measuring some RO/DI water. If that doesn't read zero, something's wrong.

 

[sillygoose]

That's a good idea. My TDS has been zero coming out of the RODI but NO3's not a big player in TDS. The sad truth is that I probably have a bunch of NO3 in my tank. My nano has less but it's still at 20+.