Pools and spa should never be filled with soft water from a softener. “Water that has a very low calcium and magnesium content (water hardness) – usually means less than 100 p.p.m. or 6 grains. Also, water that has gone through a water softener. Water with less than 100 p.p.m. of hardness should be increased to a minimum of 150 to 200 p.p.m. using calcium chloride.” IPPSA Basic Training Manual, 2006 pg 189, Glossary. Pool water is constantly trying to find equilibrium. If the pool water is out of balance it will try to balance itself by pulling out what it is missing from the pool finish or tile grout and deposit what it has too much of on pool finish surface or tile.. Filling the pool with water with very low calcium hardness water will cause the water to try and extract the calcium from the pool finish and this is can damage the pool finish or tile very quickly.