Calcium,ALK & Magneseum ?

Chris Ferrell · Feb 15, 2016

I have a question around the coral uptake of magnesium. My tank has been running for a year now and added some sps corals about a month ago.
93 cube with sump and refugium
Vertex 150 skimmer
1 Hydra 52

Calcium 385 switched from manual dosing to a pump and still dialing it in
ALK 6.7 also dosing
Po3 0.06
No3 0
Mag 1500 checked with Elos and Red Sea.
My question is if I am not dosing any magnesium why is it so high and since I started this I have never seen it drop much. I do 10gal water changes weekly using Instant Ocean, My tank seems to consume about 10-15 ppm of alk and calcium a day. My sps frags are small but I do have some montipora caps and digitata have been there since day 1 and growing pretty good. I know magnesium it not consumed at the same rate as calcium and alk but shouldn't it drop some if I'm not dosing

I hope this is enough info and makes sense. I don't post much but this is bugging me.

Randy Holmes-Farley · Feb 15, 2016

If it really is that high, it is because the salt mix has a lot and it is depleted very slowly. Usually about 1/10th or less than the calcium depletion rate.

But many folks have problems with magnesium testing and get faulty results.

Normal IO does not have that much unless the salinity is very high. But it has plenty so that unless you are dosing a lot of calcium, you won't need magnesium.