If you live in a hard water area then the water marks from tap water on your shower screen can be a perpetual nightmare. The term "water marks" is missleading. It is not the water that is leaving these marks but the impurities in the water such as lime and chlorine that are left behind when the water drys. So the water we use has gone through a 3 stage purification process to remove all of the impurities.
Normal tap water in our area has a TDS (total dissolved solids) reading of between 350 and 450 ppm (parts per million) of solid impurities. After the purification process this can be as low as 0 ppm TDS. Up to a reading of 15-20 ppm TDS there would not be any traces of water marks left behind but once it starts rising above this figure they would begin to appear.
We monitor the TDS reading and perform maintenance to the system regularly to ensure the TDS reading is kept to a minimum ensuring after we have cleaned them they dry clean and crystal clear..