Magnesium consumption?

b4tn · Apr 4, 2016

So novice reefer here learning as I go. I have been dialing in my daily dosing routine. I had it dialed in almost perfect until I added several SPS. Now my magnesium seems to be dropping. I have a 75 gallon display with a 40 gallon sump. I am using reef crystals and doing 10 gallon water changes bi weekly.

I was under the impression magnesium is a once every 6 month or so check but over the past 2 weeks my magnesium has dropped 140 ppm. I am able to keep calcium and alkalinity pretty stable and both SPS and LPS are obviously growing but should I start dosing magnesium daily as well?

Here are my current levels using Red Sea test kit and a hydrometer.

Temp 80
Salinity 1.026
Calc 430
Alk 9
Mag 1440.

I'm dosing 20 ml each of soda ash and calcium daily from BRS.

dbl · Apr 4, 2016

Your Mag level is fine where it is, if not a little high. Randy can correct me if I'm wrong, but acceptable range is 1250-1350, and a little higher is not the end of the world.

I would suggest let it ride and by what you're saying, let it come down slowly and see where it settles.

Just my opinion.

Randy Holmes-Farley · Apr 5, 2016

Your assumptions are correct.

With your addition rates, your actual magnesium consumption is likely about 2 ppm per week. Anything greatly more than that is testing error, or is due to water changes with a salt mix that is not matching the tank.

b4tn · Apr 5, 2016

Thanks, I seem to get mixed opinions from searches on Magnesium dosing. Some don't dose at all, some daily, some over long periods of time. Whats the best best practice once you figure out consumption?

Randy Holmes-Farley · Apr 5, 2016

b4tn said:
Thanks, I seem to get mixed opinions from searches on Magnesium dosing. Some don't dose at all, some daily, some over long periods of time. Whats the best best practice once you figure out consumption?

Many people with dosers dose daily, but that doesn't mean it is needed. If you drop 20 ppm in 10 weeks, that's more than often enough to maintain magnesium.

FWIW, some people are dosing a LOT more calcium and alkalinity than you are each day (as much as 10x), so have higher magnesium demand.

I never dosed magnesium to the main tank. I just boosted the salt mix that I used for daily water changes.