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Discussion Starter · #1 ·
According to my water source, the tapwater here contains 50ppm of hardness, 40ppm of which comes from calcium carbonate and 10ppm from magnesium hydroxide, giving a 4:1 ratio of Ca:Mg. This is about 3 dGH.

Is 10ppm of Mg(OH)2 easily sufficient for plants? I am adding some GH booster after water changes but I'm wondering how much I need to really add. Does the concentration required vary from tank to tank?
 

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I don't know how the information was supplied to you, but the hardness typically is given in units of ppm as CaCO3. If that's the case it ought to mean that it's not as simple as subtracting 40 from 50 and getting 10. Instead you'd have 10 ppm as CaCO3 in Mg(OH)2 rather than 10 ppm Mg(OH)2, meaning that the Mg source in your water contributes as much hardness as 10ppm CaCO3 would. That's 0.1 mmol as CaCO3 per liter, or 0.1 mmol Mg(OH)2 per liter, or 0.1 mmol Mg2+ per liter, or about 2.4 ppm Mg ions.

I find it odd that they'd give the value for magnesium as a quantity of magnesium hydroxide. How do they know it's not magnesium carbonate and calcium hydroxide, for instance? They're all dissociated anyway; it just struck me as strange.

ETA: Note, if that's not the case and you actually do know for certain that you've got 10 ppm Mg(OH)2, then that's more like 4.1 ppm Mg ions. I don't know how much you're actually looking for.
 

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Discussion Starter · #3 ·
No, I guess I interpreted it wrong. It was given to me as 40ppm CaCO3 and 50ppm total hardness. No direct ppm of Mg(OH)2 was given to me. I just thought that it worked this way. So this would mean that I would have 2.4 ppm in Mg2+ ions, like you said... that seems rather low... does that seem low for growing plants?

edit: if this still isn't quite the right way of looking at it, then what is?
 
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