Magnesium chloride?

[Frogger]

How much do you need? You really shouldn't be requiring that much Magnesium unless you have a monstrous tank.

I use the stuff from J&L and only add it when needed. Have not had a problem with it that I know. My tanks really do not use much magnesium probably because the salt I use for water changes has more than enough in it.

The Bulk Reef Supply stuff is really good as well.