Hard Water! Need advice! +rep

I'm currently in week 6 of flowering and the water I use is very hard (high in Calcium). Now, it is starting to lock-out the magnesium from the leaves.

What is a good way to get Mg back into the plant; or, how can I get the Cal out of my water? Also, it's week 6, so I can't use any really harsh chemicals or additives. I've read Epson Salt can help; but, I'm not sure what amounts to use or how often. Anyone have experience with this, any advice?

+rep for help! Thank you!

If your water is in a container let it sit out opened up to let the water break down to its basics then put like half a teaspoon of epson salt in then repeat for its next watering but dont add epson salt so its just distelled water.