Hard water is something that comes from having too many minerals in your water source. It can cause a lot of problems with plumbing if it's too hard, and most buildings will have resources for dealing with this to help avoid calcium buildup in pipes and along tubs or pools. As for how it interacts with a float tank, specifically, it seems like the larger issue is going to be how it impacts the rest of your building. 

Graham and Ashkahn break down what they know about how hard water affects float tanks and the differences you're going to have to look out for if you're using well water over municipal water sources.