dosing magnesium

My magnesium levels are low. I need to  add 300mg/L magnesium to about 190L of water.

So that's 57g of magnesium right? It just seems like a lot to me.

Can I ust add 57g of MgCl or do I need to only count the magnesium toward the 57 grams?

How quickly should I add this?

thanks!

My magnesium levels are low. I need to  add 300mg/L magnesium to about 190L of water.

Yup, 57g of magnesium.  If you use MgCl2, it's likely hydrated to MgCl2(H2O)6.  This compound is only 24.3/203.3 = 12% Mg.  So you'll need to add 57g/0.12 = 475g of magnesium chloride hexahydrate.

If you're using epsom salts (magnesium sulfate hexahydrate) it's only 24.3/246.5 = 9.9% magnesium.  So you'd need to add 57/0.099 = 576g of epsom salt.

If you would like to try to balance the chloride:sulfate ratio to seawater you'll want to add about 9 parts MgCl2 to 1 part MgSO4.  Most people do not have access to good quality MgCl2 though.

Is my salifert Mg test good enough for me to measure mag, dose, measure mag again and calculate the exact volume of water in my tank?

You might be able to do if you could verify the purity (and degree of hydration) of your chemical.

How much water will it take to dissolve that much Mg coumpound?

I'm curious why Deedo figured he's that low in Mg. Are SPS doing poorly? And how are the levels of other trace elements, like Sr and Mo?

So I have to add half a kilo of that crap! How slowly should I add it? Will this significantly alter my salinity?

Over half of the weight of either of those salts is just water.  A one time dosage won't raise you salinity much.

I think it almost always surprises people when the learn how much "salt" is needed to raise magnesium levels.  It's mostly due to the fact that magnesium salts then to be heavily hydrated and magnesium itself is almost half the weight of calcium.