doesn't look like anyone will give you a more scientific answer so i will tell you what I know about hardwater in general. Hard water can refer to two things and likely it is referring to a higher ppm count in your water. Some places have a hard water that is full of iron? I think or some other mineral that tends to come up from wells and what not. Up north people will use a water softener/salt allowing them to better use their well water. This would be for general usage that I am speaking of and not growing.

What you are concerned with is a higher ppm count in your water also called hard water. A hard water nute, would simply have less ppm's of calcium and magnesium if I recall correctly as these are the common minerals found in higher ppm tap water. I think it will work, but what concerns me is that you don't really need it and as such you may under feed your plants in certain minerals. A 40 ppm tap water will be short on cal/mag right? It would almost be as if you were using RO water, right? Normally you would be putting in cal/mag (branded additive for RO water) to bring your ppm up to about 100 to 150 or so. You wont be doing that here though. You are just adding in your nutes designed for hard water or in other words adding in less ppm of nutes then what you should be. As such you may find your water lacking in cal/mag which could cause issues for the plants pretty quickly.

In short keep a close eye on the plants and see how they react. You may find it necessary to purchase a cal/mag additive to supplement your lower ppm water and nutes. I don't believe adding more of your nute solution would resolve your issue either as all you would be doing is adding in more NPK and not C&M which is found in most tap water and non hard water nutes. Maka sense to you?