House,
I regularly test Mg levels (once every two weeks) and find that it is key in keeping everything stable, i.e. pH and ALK. Low Mg levels will cause large pH swings (especially when using a Ca reactor) and ALK to drop.
It is probably the most overlooked component in the pH/ALK/Ca equation. Interestingly, it is lacking in a lot of the different salts that are available. EDTA, which is added as a preservative in different salt mixes chelates Mg, i.e. binds and renders it unavailable for enzymatic reactions (this is how the salt is "preserved"), further limiting the amount of Mg present in our tanks.
Additionally, no one really knows the amount of Mg used during coral growth, but it is involved in all living enzymatic reactions as a co-factor. I am sure that it is used by corals to grow, further limiting its presence in our captive reef environment.
Anyway, sorry for the draw out hot air which you probably already know. Now that you have all these SPS, how about sharing the wealth,
. As a trade, I test your Mg levels for frags!!! save you the test kit purchase
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