Alkalinity dropping with stable Ca and Mg

mp2022 · Feb 17, 2026

Hi all, I have a 48 gallon tank with a mix of mostly SPS and LPS and a few soft corals. It’s about a year old now, and most corals, particularly montis and LPS are growing fast and very well. I have a few acros that are very slowly encrusting at their base and have ok polyp extension.

My alk had been hovering around 7.8-7.9 for about 6 months, with Ca around 420 and Mg around 1230. Got kind of lazy with testing and didn’t test alk for about a month and the alk is now around 7.0, but Ca/Mg grossly stable. I dose all for reef, and in response to this have increased daily dosing about 10-15%. I do a 15 gallon water change about every 3-4 months.

The questions I have for the group are:
1) What is the significance of Ca/Mg remaining stable with the dropping Alk? I am assuming it has something to do with coral growth, but I would also expect Ca/Mg to drop but I am not a chemist.
2) What are the best ways to correct this without messing with the Ca/Mg too much and how quickly do i need to do this?

Thanks in advance!

get-salty · Feb 17, 2026

Coral growth for sure and since you are using one part AFR, you need to do manual adjustments of two parts - alk / cal to stable it out.

NeptuneSpear2011 · Feb 17, 2026

Hey @mp2022 First question I'd probably ask is how you're dosing all of your elements currently and then I'd want to know how much and how often during the day?

Usually (depending on the bio load or amount / type of corals) if you have a high amount of corals and they're growing fast that tells me that your corals are healthy and probably consuming Alk at a higher rate than Ca, Mg. Mg isn't usually consumed as quickly as Ca/Alk; stony corals consume Ca higher than soft corals(mushrooms, etc.) to build and grow the skeleton base and new extensions. But this is also dependent on PH levels too.

In short, you're probably consuming Alk quicker due to growth and may simply need to tweak or up the dosage amount.

Hope that helps

Randy Holmes-Farley · Feb 17, 2026

With that alk drop, you would not expect to reliably detect the expected calcium drop of less than 10 ppm and cannot detect the expected mag drop of less than 1 ppm.

I would dose calcium and alk in the expected ratio.

Pod_01 · Feb 17, 2026

Just additional information that may be useful, calcification consumes major elements in following ratio 2.8 dKh/ 18-20 ppm Calcium/ 1-2 ppm Mg.
AFR supplies Alk, Ca and Mg in this ratio so as long as your Alk is maintained you really don’t need to measure Ca or Mg. The Ca in AFR is on the higher range so over time it can creep up slowly, but since you are doing water changes it will be a slow increase if any.

For example I use ICP tests to monitor Ca or Mg and those are done once or twice a year and in 4+ years I didn’t need to adjust either.
Titration test kits from my experience for Mg are tough to use and same can be said for Ca kits.