Hydroponics, as a system of growing based upon water (just look at the etymology) is understandably based upon H2O. Ultimately the quality and temperature of your water is important if you want to raise healthy plants and bumper crops, both will be explored below.
Most water in our houses is affected by calcium and magnesium levels, too much of which can cause hard water and limescale (limescale being a deposit including calcium carbonate) which in turn is pretty bad for your plants. Fortunately to find out the levels of salt, magnesium and calcium in your water it is possible to buy testing kits, or even send a sample away o a laboratory.
If you do find that you have excess hard water and this is harming your plants, restricting their ability to withstand and recover from disease, you can treat your water with softening agents specifically created for hydroponic systems. Another option is to collect rain water in a butt or to use a filtration system. Commonly however, many gardeners advice that using spring or mineral water should be avoided as this can actually end up poising your plants by unbalancing the nutrient solution.
Also important is the temperature of your water as if it is outside of a specific range the plants can suffer. Typically a temperature between 18⁰C-26⁰C (65⁰F-80⁰F) is suitable, it should be remembered that before adding new water, leaving it out to reach the same temperature as the water in the reservoir is advisable as sudden changes in temperature are not good for plants. In order to keep the water in your hydroponics system in this range it may be worth buying an aquarium heater for winter months and even a cooler for the summer (although in the UK this is probably less of a problem!).
Both water quality and temperature are key to giving your plants a nutrient solution that they can use effectively to grow and flourish, by understanding these principles it should be possible to have an efficient hydroponics system.