Magnesium in the Reef Aquarium

Codeman00

Codeman00

Premium Member
Randy..

I have a 75 gal tank (100 gal total volume) that has a small clam, some softies, and LPS, but is dominated by SPS. I've read most of your articles on recommended reef parameters and also ones that specialize in calcium, alkalinity, pH and magnesium.

During my initial research on SPS tanks, I knew that I needed to keep track of calcium and alkalinity and I test for them all the time especially when setting up my calcium reactor. I then had a low calcium level issue early on and I realized it was because my magnesium was low. It seems like everyone concentrates on calcium and alkalinity, but not on magnesium and many articles lead us that direction. It led me to assume that magnesium levels should very slowly fall so I never tested for it that much. I'm amazed how much magnesium I lose in a given time. In a 1 1/2 month span, my Mg level might go from 1300 to 1150 ppm. Is this normal? It just took me by surprise because no one really preaches that magnesium is so important to test for yet it the levels seem to drop much quicker than I expect.

I realize that my new IO saltwater might be low in Mg too and that frequent water changes could hurt my Mg levels. I have never tested IO for Mg levels, so I'm not sure.

I just wanted to get your general thoughts on this. Is my tank normal?

Thanks in advance.
 
In a 1 1/2 month span, my Mg level might go from 1300 to 1150 ppm. Is this normal?

In a reef aquarium with no magnesium additions, the magnesium drop ought to be on the order of 6.5% of the calcium incorporation (plus or minus a pretty big factor as different organisms use different amounts).

In an SPS heavy tank, one can easily need 10-20 ppm calcium per day, or 450-900 ppm in 45 days.

The magnesium demand could then be as high as 25-50 ppm in that time.

You are suggesting it is 150 ppm. Maybe, especially if the demand is very high in your aquarium.

BUT, many salt mixes are low in Mg++, and that may also contribute to your drop. I'd measure the magnesium in your IO. I add magnesium and calcium to mine. :)

I discuss magnesium issues in this article where I justify the amount to add to my DIY system:

An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System
http://reefkeeping.com/issues/2006-02/rhf/index.php
 
That's great information, Randy. I will definitely read that article and test my water change water.

But the fact still remains that many articles and RC posts act like magnesium is just a sideshow ion and that calcium and alkalinity are so much more important when in fact, they really aren't. I was just under the impression that I didn't have to worry about Mg levels that often. Boy was I wrong.

As always, thanks again for help.
 
I agree that it can be important. What exactly happens when it gets low isn't as clear as what happens when calcium or alkalinity get low, however. :)

How important it is to closely monitor the magnesium depends a lot on the type of aquarium, the amount of water changes, and how calcium and alkalinity are supplemented (since some methods supply magnesium and some do not).

Happy reefing. :)
 
I recently have been monitoring the mg on a 4 day cycle. I have made some very good observations. At a level of 900ppm algae growth is very limited. As the number is increased to 1050ppm low or non-calcifying algae start growing strong. As soon as my tank hits 1100 ppm coralline algae grow at an alarming rate. A white edge can be seen around the perimeter of growing coralline. The mg level seems stable until it hits this 1100 ppm mark. At this point the tank consumes 50 ppm every 4 days. I have been adding Seachem Reef Advantage Magnesium in DI water, every 4th day to get the level up. Unfortunately, I haven't been able to get it past 1150 ppm, but am still trying.

I recently read that epison salt can be used to increase the mg, What happen when the sulphate levels get to high?

The tank I am talking about is 4 month old, 240 gallon main w/ 110 gallon refugium, 350 lbs live rock. A few LPS and SPS, shroms and zo's
 
I recently read that epison salt can be used to increase the mg, What happen when the sulphate levels get to high?

At some point it becomes a tox concern, but I do not know exactly what level that first starts at. A better bet is MAG Flake from Home Depot (magnesium chloride from the Dead Sea Works Company), or a commercial magnesium supplement such as from ESV or Kent (not Seachem, I would avoid Seachem magnesium as it is no better than Epsom salts, IMO).
 
Sorry if this sounds ignorant, but I've been reading that DIY supplement system and I believe it says the 3rd component uses both MAG flake and Epsom salts. Is that the case? This thread makes it sound like you use the first in place of the second.

Just asking for clarification! Thanks for this resource!
Clint
 
That is a good queston. :)

The original article used just Epsom salts, so in that sense the MAG flake plus Epsom salts has replaced just Epsom salts.

Also, if you are not using calcium chloride (and so do not already have a big chloride boost from that), then using magnesium chloride (MAG Flake) alone to boost magnesium is not a problem.
 
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