Cant keep cal or alk up

Jenkins...I have the same issue. Two weeks ago my Ca=450, Alk=8.5 and Mg=1440. I went out of town for two weeks and tested again. Ca=300, Alk 7.7 and Mg=1440. Before my trip, i also constantly dose Ca and Alk but my Mg is stable at 1440. pH stays around 8.0-8.2 and salinity 1.027. I use Kent salt change 10% every week to 10 days. I have recently had a lot of deep purple coraline algae growth. Wondering if the algae growth is the cause?

 

I'm not quite sure I quite understand your comment as written.

Magnesium and phosphate bind to calcium-carbonate and disrupt the crystalline structure. This prevents more ca and bicarbonate from precipitating. If mg and PO4 are low there will be more calcium and bicarbonate precipitated as calcium-carbonate. Dissolved ca and bicarbonate do not have any direct effect on magnesium. They do have indirect effects though though precipitation, either abiotic or biotic i.e. calcium carbonate precipitation, or growth of organisms.

As alkalinity drops, pH will drop. At some point precipitation will slow down and dissolution of calcium carbonate will speed up and the system will tend to reach an equilibrium.

Also, to clarify, Mg, is used significantly more if there is a large bioload than if there is a small one. However, mg is only used at a fraction of the rate of calcium and bicarbonate. Also, mg levels are kept much higher than ca and alk, and test kits are not not particularly sensitive. So, generally the consumption is still fairly undetectable compared to ca and alk consumption. I meant virtually unrelated within the detection threshold. Reading my comment again I think that was unclear.