hard to keep magnesium high

[tacking]

My tank is 50 gallons. Calcium and alkanity are relatively stable with little amount of supplements, i.e. ca=420 ppm & 7 dkh. But the consumption of magnesium is surpisely high. I raised the magnesium level to 1350 ppm and checked it two days later. I found the magnesium level dropping to 1250 ppm. Is consumption of magnesium in a new tank where there are only live rocks and a shallow sand bed such high?
I'm afraid I have to pump much more magnesium supplement when my tank is stocked corals later.:confused:

[m2434]

This is a new tank? Possibly set up with fresh "dry rock" and "dry sand"? Perhaps if there is a lot of fresh aragonite for it to bind to, it could drop quickly. If so, the mg should level off after a little time. However, otherwise it dosn't make sense. Most mg test kits seem to be junk, so, I'd guess testing error.

[m2434]

Here is an article on mg:
http://www.advancedaquarist.com/2003/10/chemistry

[tmz]

Coraline uses it too but a true 100ppm drop in a day is unlikely,paticularly without a drop in alkalinity . Perhaps it's testing noise or related to a water change with low magnesium salt.. FWIW I have an sps dominant system ;use a high magnesium salt mix for waer hanges and never hae to dose magnesium to maintain >1400ppm .
I

[tacking]

My tank is setup for about 3 weeks only. The rocks are not dry as they are live rock. I agree with you that the sand provides many surfaces for magnesium to precipitate. I use the same test kit and test magnesium frequently. So the accuracy of the test kit may not be a big problem because I only compare the results. The absolute value is not the point I most concerned. I just wonder why there is drastic drop in magnesium compared to that in calcium and alkanity as calcification uses up much more calcium and alkanity than magnesium.

[bertoni]

It must be a testing issue or some similar problem. Magnesium can't precipitate without taking a lot of alkalinity out of the water.

[Randy Holmes-Farley]

I agree that the 2 day change is testing error. There is no way for it to actually decline like that.