How important is water quality in hydroponic growing?

It is vital. It is the hydroponic substrate par excellence.
The hardness of the water (hard water contains a high level of calcium and magnesium) must be taken into account when choosing nutrients. There are nutrient formulas designed for hard water and we recommend that you use these if necessary. To find out about the quality of your water, contact your water provider who will provide you with a water analysis free of charge.
If your analysis shows a calcium content of greater than 70ppm (mg/litre), use TriPart® Micro or DualPart® Grow (hard water) formulas. To ensure the quality of your water, you can also use rainwater or install a reverse osmosis filtration system. In these two cases, mix approximately a quarter-litre of tap water with your filtered water to restore its buffering capacity. Do not use mineral water as it destabilises the nutrient solution and may even be toxic to the plants.
In France, water hardness is measured in TH degrees.