Triton Magnesium Chloride - 4 Kg.

Magnesium Chloride Hexahydrate MgCl2.6H2O

What does Magnesium do in my aquarium?

If you are having swings with Calcium and Alkalinity there's a high chance your magnesium is low.

Without magnesium, you would not be able to hold the proper amounts of calcium and alkalinity, and they would simply fall out of solution. Magnesium is essentially a buffer between calcium and alkalinity and is slowly consumed by corals, algae, and invertebrates, and in most reef tanks magnesium will need to be supplemented over time. 

Instructions: 

• Test your aquarium's magnesium level.
• Determine the amount of Magnesium Chloride powder needed to dose to your aquarium by using the chemistry calculator (link below)
• Dissolve the dry powder in RODI water.
• Once fully dissolved, slowly pour the solution into a high flow area of the sump, overflow, or display.
• Wait a few hours and test the magnesium level of your tank.

Reef Chemistry Calculator

 https://www.triton-lab.de/en/calculators/diy-salts