Hard water is not really a problem. I had well water with 21 degrees of hardness and plants grew amazingly well in that tank. Best plants I've ever grown really.
DI is one way to alter the ratio, but so is adding Ca and stopping any Mg addition. Before you start changing Ca:Mg ratios though, why not take a water sample to the LFS and have them test it for free. Ask them to do a Ca test and a GH test. We can calculate what the Mg is from those two numbers. You may not have a problem with Ca:Mg at all.
Also, the curling you are seeing isn't quite bad enough to really definitively identify it as a deficiency. More time is needed to say for sure.