If you can find it at a local hardware store DowFlack calcium chloride (sold as a de-icer) is what you need to raise calcium hardness. On the Dow website there is a page on pool and spa use of DowFlake for this purpose. It is exactly the same chemical that you pay more for at the pool store as calcium hardness increaser! (Dow is one of the larger manufacturers of calcium chloride and much of the stuff you buy at the pool store is probably repackaged DowFlake. BTW, it is used for much more than just de-icing but that is the most commen use for it when a hardware store stocks it.)