magnesium - how high is too high?

Discussion in 'Water Chemistry' started by JJK, Aug 17, 2009.

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  1. JJK

    JJK Teardrop Maxima Clam

    Mar 7, 2009
    I just switched salt mixes - I had been using Instant Ocean, but was finding my calcium and magnesium levels too low. So I bought some Seachem Reef Salt, and did a 25% water change. My calcium is now at 460 (had been at 400), but I was most surprised by my magnesium. It used to be around 1100. Today I ran a Red Sea Mg test, and it was off the scale (although the scale ends at 1300). I kept adding reagent, and would estimate the Mg level at around 1600.

    So.....what do I do? Are levels like this detrimental to the tank? Is this significant? Advice, please.
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  3. Night-Rida

    Night-Rida Finback Whale

    Jul 3, 2009
    Tampa, FL
    Ive read somewhere.. you cant 3x times mag to calcium levels..

    so cal at 450 then mag at 1350 or 500 cal / 1500 mag with a salinity of 1.025-1.026

    I wouldnt worry about it.. just boost a little more calcium.. get it to 500
  4. ReefSparky

    ReefSparky Super Moderator Staff Member

    Nov 27, 2007
    South Florida
    As far as I know, mag can be safely high without detriment to either parameters (like calc or alk), or livestock. If it's too low, then you'll have problems getting calc or alk up to desired levels, or perhaps you'll only be able to get either calc high, and alk low, or the converse--high alk with low calc. I'd allow it to come down naturally over time with water changes, if you do water changes.

    The risk of raising your calcium is you're doing so at the expense of your alk. If your calcium is raised to 500, for example; you may find that you can't get your alk up to desired levels. Another downside to elevated calcium is a greater probability of precipitation, and almost certainly over time; a buildup of scale on hot surfaces (motor impellers, heaters).

    IME, when juggling these 3 components, shoot to get your magnesium right first--I go for 1500, but 1350 or thereabouts is acceptable. Only then--providing that your salinity is 1.026--will you easily be able to get calcim and alkalinity in balance. FWIW, I like Salifert test kits for these 3.
  5. Crimson Ghost

    Crimson Ghost Blue Ringed Angel

    Nov 14, 2008
    Somewhere south of disorder
    Magnesium is roughly 5 times more abundant that calcium in sea water when considering the mass of ions present.. It is substantially lighter than calcium in terms of weight (Ca=420ppm, Mag=1285ppm)

    Your question, is 1600ppm mag too high – no, probably not. Magnesium shock is used in shucking oysters, they pump the Mag to 8000.

    Of course the target should be natural seawater at 1285 but generally speaking 1250-1350 seems to be the norm for reefers.

    I am concerned for your 500ppm jump in one day – Mag should not be elevated more than 100ppm per day. I’d also suggest to let it adjust slowly through water changes and perhaps throw some speculation towards the test kits.

    I am still drinking the morning coffee (on vacation so it early here)

    Lets delve deeper into the effects of Mag on Ca and Alk.

    Calcium carbonate is supersaturated in seawater, calcium ions interact with carbonate ions and form calcium carbonate (the stuff that makes the wheels on our tanks spin)

    Calcium and carbonate would precipitate out if either are elevated to high, Magnesium acts as the buffer permitting both calcium and carbonate to rise higher than they could otherwise (the dreaded snow storm effect if you will)

    OK – so that’s why you will have trouble raising calcium if your Mag is low. Ack – kids are now awake….gotta run. (remember – on vacation so its all about to get going over here).
    Last edited: Aug 21, 2009
    1 person likes this.
  6. unclejed

    unclejed Whip-Lash Squid

    Sep 23, 2008
    Clinton Township, Michigan
  7. Dr.Fragenstein

    Dr.Fragenstein Panda Puffer

    Oct 30, 2008
    SE Wisconsin
    To add alittle as its been touched enough now... Crimson hit it on the head, the Mg will actually prevent the Ca and CO3 from further forming CaCO3 and dropping out of solution... Not saying you don't want CaCO3 formed... you don't want it formed in the water column, you want the corals to be forming it.

    As far as concentration of Mg... Most people shoot for 3.25 the amount of Mg in their tank as compared to Ca concentration. I personally shoot for slightly over NSW in my tank at home...

    Elevated Mg can be a good thing... It is at the moment the only known, effective way to rid your tank of bryopsis... Many people will take the Mg up to 1600ppm but I have found that anything in the upper 1400's is usually high enough to be effective.

    In your case JJK, which I am surprised no one asked is how big was your waterchange? You have to look at a water change in simplicity sake as a weighted average... If you Mg in the tank was 1100, and your tank has 90 gals in it(probably not but using the sumps water to displace the rock...) and your new water has X ppm of Mg... how many gallons of water would you have to change or how RIDICULOUSLY high would the Mg in the new salt need to be to bring it up 500 points?? I have a feeling the test was wrong...

    Ex lets say you changed 20 gals... so 70(1100)+20(x)/90=1600? sounds goofy to me?