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RE: so how much magnesium
Mark - i'm going to answer your post 'cause i've had a similar situation - medium hard water, almost all of it calcium. doing the calculations for hardness is especially annoying because everything is usually actually reported as ppm CaCO3 rather than Ca++ or Mg++. check your test kit to see if this is so. At any rate, i'd say you should probably start by adding about 10ppm. that is, 10ppm Mg++ as CaCO3 :). makes sense, right? right. you could probably add three or four times this much to bring your Ca/Mg ratio to the 3:1 that has been espoused but if you want to be conservative about increasing GH 10ppm should show improvement if any improvement is to be had. so here goes:
You have 160 liters of volume. You want to add 10ppm (of whatever). That’s 10mg/L times 160 liters = 1,600 mg = 1.6 g total. If you were interested in adding 10ppm Mg++ this would be pretty straightforward as Epsom salt is about 10% Mg++ by weight, so to add 1.6g of Mg to your tank you’d add 16g of Epsom salts (10g of granulated “Good Sense” brand Epsom salts is about 13mL in my experience – I don’t know what that is in teaspoons).
You want to add the equivalent of 10ppm CaCO3. so, 10ppm is still 1.6g in 160L. CaCO3 is 40% Ca so 1.6g contains .64g Ca. now, Mg weighs only 60% as much as Ca so the equivalent amount of Mg weighs .38g. The amount of Epsom salt containing this much Mg is 3.8g. Or simply, as you can see this amount is about 1/4th the amount above for adding 10ppm Mg because CaCO3 weighs about 4X as much as Mg.
In the end
Add 3.8g Epsom salt to your tank to add 10ppm. This is about 5mL, which I think is a teaspoon.
Remember not to add more than this without doing a 50% water change – this will prevent the possibility of raising the GH by more than 2X the target level.
Hope this is helpful