Alk dropping 0.5 every day but Ca and Mg remains at 450 / 1450

[SoCalVictor]

I have a similar situation to another post on this sub-forum but with some differences. Previous post did more water changes that accounted for Alk drop. I am noticing these Alk changes daily, even without water changes. In fact, I use Instant Ocean so the Alk is actually a bit higher than what I maintain at 8.

As the title states, I'm dropping 0.5 dKh per day but my Ca and Mg remains steady at 450 / 1450. The tank is relatively 'new'. Quotes because I had transferred established live rock and let it sit for 4 months. The sand is medium grit aragonite and was new and sat with the live rock for those 4 month. Then I slowly transferred over the corals, including sps, lps, and mushrooms. The corals came from a 65 gallon system and was growing at a healthy rate with proportionate Alk, Ca, and Mg consumption.

What's currently happening with Alk consumption but Ca/Mg remaining steady has been going on for almost 6 weeks, tested with both Salifert and Trident. I'm maintaining the Alk levels using the BRS mix. There is some growth on the sps and all the corals look happy. Some corraline but not as fast as was happening in the 65g system. I also have clams in the tank, 2 maximas, 1 squamosa, 1 deresa, and 1 gigas. The tank is lit by a 8x110w ATI T5 fixture.

Any insights into what may be happening?

 

[Randy Holmes-Farley]

Sure. With a drop of 0.5 dKH per day, the calcium drop (about 3 ppm) is too small to see reliably in less than a week or so. You won’t detect magnesium consumption (less than 0.5 ppm) in less than a month or so.

Additionally, other factors, such as increasing nitrate, will deplete alk and not use calcium.

 

[SoCalVictor]

In my 65 gallon, I was also dropping around 0.5 dKH a day but noticing a measurable calcium drop, 10-15ppm which translated to about twice as much ml of Ca then Alk in dosing. Any reason the Ca consumption was more significant before?

In my current setup, I have not transferred the fish over so the bioload is small. My nitrates are fairly low and remain constant.

 

[Jeeperz]

I drop more than that alk each day in my 20 gallon and after a week see no measurable drop in calcium or magnesium

 

[Randy Holmes-Farley]

The biggest reason is inaccurate testing, followed by water changes. Other effects are usually very small (except sulfur denitrators) but can seemingly be significant when the absolute demand for calcium and alk is low.

this article lists all the possible ways demand can get skewed:

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​ ​ When Do Calcium and Alkalinity Demand Not Exactly Balance?​

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