Magnesium usually makes up about 1/3 of the total hardness of most water. It is found in many minerals, including dolomite and clay. It is very abundant in sea water--usually five times the amount of calcium. It is seldom a significant problem, but it is removed along with calcium when hardness is treated.

Treatment: Magnesium is readily removed by cation exchange (water softener). Reverse osmosis also removes magnesiuim.