Water hardness?

[HAYMAN]

What do you add if your water is showing too hard?

[waterbear]

softer water (not joking here) . If your hardness is too high you need to drain and refill with softer water. If that is not possible there are sequestants that help reduce the hardness of the water by seqestering or chelating the calcium in a similar way that metal sequesterants do. Calcium is, after all, a metal.
Try googling calcium hardness reducer pool.