Chemistry Madness - Different Kits Yield Radically Different Results

I apologize in advance that I seem to be starting threads on multiple topics in different forums (some of which are inevitably linked).

Real quick - I inherited a tank from my cousin, which is a very fully developed 6 year-old ecosystem with corals, fish, shrimp, snails, etc. Everything looks quite happy, aside for some critter that seems to be taking a small chunk out of my acropora and my historically finicky scolymia, which may be receding a little (I am told by my cousin that this is a very difficult coral to keep happy). I did have a bit of an algae build up last week, but it was mostly on the glass (and I was on vacation).

Here is a summary of my issues:

Nitrate:

1) I inherited an API kit from my cousin which read between 20-40 ppm for the first few weeks after I moved the tank. A few weeks after moving the tank, I thought the nitrate should have been dropping (and knowing the API kit was very old), so I bought a Nutrafin kit.

2) The brand new Nutrafin kit has read consistently at < 5 ppm nitrate since I got it. Since it seemed to me the nitrate levels should have been under control and everything pretty much looked happy, I figured this was the accurate test.

3) I also have an API test strip which, although it's very difficult to read, registers about 20 ppm nitrate.

4) I just bought a new API Master Marine Kit as well as a Master Reef Kit. Both come with API nitrate tests, so I figured I would test just for comparison. One of the two brand new API kits I bought registered 80 ppm while the other registered 160 ppm nitrate!

So I have one brand new Nutrafin kit telling me the nitrate is near 0 and another kit that says my nitrate is out of control! And yet another nitrate test strip by API that tells me nitrate is fairly low. Having run these tests, all I can say with confidence is that my nitrate concentration is a positive number between 0 and infinity (or, to be fair, the saturation concentration).

Calcium:

1) I have an old API kit, again inherited from my cousin, that yields 580 ppm Ca2+.

2) My brand new API Ca2+ kit is showing 740 ppm Ca2+. Is that even possible? I'm not sure what the saturation concentration of Ca2+ is for the pH in my tank, but that seems extreme. Not as crazy a difference as the nitrate, but still pretty large.

Alkalinity:

1) Tetratest kit reads 8-9 dKH.

2) API test reads 12 dKH.

That's a pretty significant difference.

For reference, here are some other values in my tank (assuming THOSE kits are even accurate):

pH: 8.2 - 8.4 (different between 2 different API kits)
NO2-: 0 ppm (consistent between 2 kits)
NH3: 0-0.25 ppm (different readings between 2 different API kits)
Specific Gravity: 1.025
Phosphate: 0.5-1.0 ppm
Mg: 1540 ppm

I have been doing regular water changes, running my skimmer (which is well-tuned), adjusting alkalinity, running my UV sterilizer, and generally keeping on top of all the numbers. I asked my cousin about the numbers, and he said he was hyper-vigilant about them in the beginning but then just went off how the fish/coral look. Generally, my fish and coral look pretty happy, so I'm inclined to believe the API nitrate number is simply not correct.

That being said, there appears to be enough variability in these test kits as to render them almost meaningless. I really have no idea what to make of the nitrate results. I suppose I could take my water to the local shop, but they are just going to run yet another test kit. I can imagine 10-20% variability, but having almost completely different answers on the nitrate is truly perplexing. Which results to believe??

 

There's a lot to address here, I'll start with the nitrate issue. This is just my opinion, and others may say otherwise. You know that your acropora has been having separate problems (getting stung, alkalinity swing, and some pest) along with your Scoly which you know to be finicky, coupled with an alk swing (according to a previous thread). If these are the only corals showing problems, both of which have known causes, you can safely assume that your nitrates are not significantly high, and that the aforementioned things are what's making them not do as well.

If I'm correct, calcium concentration does not necessarily depend on pH, but rather alkalinity and salinity. However at 500+ ppm, your alkalinity would be much lower, or your tank would have precipitates forming.

I would scrap the API kits, I only use mine as a back up against my Red Sea pro kits.

Magnesium is also pretty high and not balanced with your calcium. What test kit did you use for this?

Time for some simple questions that may have just slipped your mind...
Are you sure you're doing all the tests correctly?
Do you hold the results up to a white light?
Are you color "blind" in any way shape or form?
How well do you clean test tubes? Leftover water can really impact readings especially for pH and alk.


I've never heard of nutrafin or testratest. Maybe someone with experience in these can help you.

As for the API, their liquid tests are usually reasonably accurate, I've never really had any readings differ too much.

 

Salifert,Redsea, and Hanna meters are good quality kits that will give better/more consistent numbers.
While these cost more it is worth it. I like the Hanna meter for Alk. Redsea for calcium and Salifert for Mag. If you have nuisance algae you have excess nutrients. You could get a 0 nitrate reading and have a tank full of algae.

 

Salifert,Redsea, and Hanna meters are good quality kits that will give better/more consistent numbers.
While these cost more it is worth it. I like the Hanna meter for Alk. Redsea for calcium and Salifert for Mag. If you have nuisance algae you have excess nutrients. You could get a 0 nitrate reading and have a tank full of algae. Don't get caught up in the numbers.

 

Thanks for coming to my rescue with such thoughtful insight, Va Reef. I really appreciate it. My sense it that the API kits don't seem accurate.

To answer your questions:

1) I used a Salifert test kit for the magnesium

2) I majored in chemical engineering and have spent my career as a meticulous numbers nerd. I'm pretty detail oriented, so I'm fairly certain I have performed the tests exactly as prescribed.

3) Yes, I hold the results up to white light, though in the case of the API nitrate test, it's pretty much tangerine orange, so there isn't much ambiguity.

4) No color blindness.

5) I clean them meticulously with fresh water (several rinses for the vials and caps) after each use. When sampling, I flush them a few times with the water to be tested, so they should be pretty accurate.

 

Do you generally only test alk, calcium, and magnesium? And nitrate/nitrate/etc only when there's an issue of some sort?

 

I test calc and alk weekly to twice a week, magnesium once every two weeks.

I no longer check for ammonia nitrates or nitrites as its only useful during the cycle IMO. The way I see it, I go by my coral. Unless a ridiculous amount of food is in the tank or my fish a suddenly die, I see no need to check those three. If my tank looks off then I would assume it's a calc/alk problem before a nitrates/nitrites/ammonia problem and even then I'd just change water.