Why is my alk being consumed but not calcium??

[Spectral_Ranger]

I recently moved my sump down to the basement and stopped dosing (BRS 2 part) for a few days to get everything resettled and recalibrated. This is a 100g system. Alk was 8.2 and Ca 460, so I manually dosed Alk and brought it up to 11.0 over two days (oops). Ca dropped to 440 over those two days. I let Alk go without dosing and it dropped 1 dKH in a day, but the Ca was at 450. For the past few days, I've been dosing Alk to keep it stable at 9, but haven't dosed any Ca. For the past three days, Ca has been 450, 470, 460. Since I haven't dosed Ca, I attribute the variation to measurement error (using Salifert). pH varies between 8.1 - 8.3 daily (per Neptune probe on Apex). Strange thing is that my Mg was a bit low (1180) a few days ago, so I've been dosing about 100mL per day to bring it up but it's staying very steady at 1180/1170 for the past three days.

So, in short, system seems to be consuming Alk somewhat normally, not consuming any calcium, and apparently consuming lots of magnesium. What the heck is going on?!? Any tips appreciated. My nitrates are at 5 and phosphates at 0.2. Corals haven't been growing as much lately and colors dulled a bit, which is partly why I was looking to reset and recalibrate all my dosing. But they otherwise seem healthy and not upset.

 

[Vinny Kreyling]

Get your Magnesium up over 1350, on my old tank that limited coral growth.
Present tank also uses mag.

 

[amcvay1979]

I agree, I keep my mag closer to 1400 and my tank doesn't use ALK or CAL in a way I need to dose to keep it reading steadily at 9.0 and 420 respectively. Must be my rock and sand is releasing it, but either way I'm happy. Pick a number to shoot for on all 3 and stick to them and you'll have success.

 

[Spectral_Ranger]

Thanks guys. I'm dosing mag heavy to get it back to 1350'ish over the next couple days. Will be interesting to see if calcium usage takes off then.

 

[bertoni]

1170 ppm is fine for magnesium, in my experience. I ran tanks for years at 1100 ppm because the older IO formula came that way. In addition. low magnesium would lead to consumption of both alkalinity and calcium as calcium carbonate is formed.

I suspect what you are seeing is noise or outright errors in the measurement process, or water changes. This article might help:

http://reefkeeping.com/issues/2004-12/rhf/index.htm

 

[Spectral_Ranger]

Guess what? I got my numbers where I wanted them, and now hardly any Alk is consumed and lots of Ca is consumed. Bizarre. I'm dosing nearly 150mL Ca/Mg per day (using Aquaforest Balling method with Ca and Mg together) and probably 5mL KH buffer. This was happening before I switched from BRS 2-part to Aquaforest, so it's not just that change that's causing it. My alk is now consistently at 10.4 with virtually no dosing to keep it there. I'm seeing good (but not great) coral growth, and like I said Ca is now being consumed rapidly. Where's the carbonate coming from???

 

[bertoni]

Hard to say. The more common stealth sources of carbonate are tap water and pH buffers.

 

[Spectral_Ranger]

Neither of those apply in this case...

 

[bertoni]

No idea, but I wouldn't worry much. It's generally not worth tracking down such issues. I might get a second opinion on the test kits, if that's not too difficult.

 

[tmz]

Some amounts of carbonate alkalinity may be consumed outside the usual calcium carbonate consumption process(,wherin calcium and carbonate are consumed @ about 20ppm calcium per 50ppm /2.8 dkh carbonate ) but that would be very small unless you are using a sulfur denitrator , for example.

 

[Spectral_Ranger]

Hmm, thanks guys. I'll try not to worry, just seems odd when I thought Ca and Alk uptake were supposed to be proportional. I did happen to get a new Salifert KH test kit because my old one was running low, and they both gave nearly identical results. While I only have one Ca test kit (Salifert), it gives expected results when I test once, dose a bunch, then retest and the level is where I'd expect it to be (same with Mg). So it doesn't seem like test kits are the issue.

 

[bertoni]

My guess is that the consumption is in the standard ratio, but various testing and other limitations are hiding that. We could try to track everything down, but it might take some time and a lot of testing. Animals can't consume calcium alone: it'd have to be deposited as a very reactive solid metal for that to happen.

 

[tmz]

I recently moved my sump down to the basement and stopped dosing (BRS 2 part) for a few days to get everything resettled and recalibrated. This is a 100g system. Alk was 8.2 and Ca 460, so I manually dosed Alk and brought it up to 11.0 over two days (oops). Ca dropped to 440 over those two days.

Likely some abiotic calcium carbonate precipitation occurred dropping the alk and calcium at that point.


I let Alk go without dosing and it dropped 1 dKH in a day, but the Ca was at 450. For the past few days, I've been dosing Alk to keep it stable at 9, but haven't dosed any Ca. For the past three days, Ca has been 450, 470, 460. Since I haven't dosed Ca, I attribute the variation to measurement error (using Salifert).

Testing noise sounds right 10ppm calcium would precipitate with about 25ppm carbonate / 1.4 dkh.



Strange thing is that my Mg was a bit low (1180) a few days ago, so I've been dosing about 100mL per day to bring it up but it's staying very steady at 1180/1170 for the past three days.

100ml of what exactly? It can take a lot of magnesium to raise it noticeably in 100g ;check this calculator:

http://reef.diesyst.com/chemcalc/chemcalc.html


If you are doing water changes the ratio of calcium and alk in the salt mix will play into it too.


.