Water should be clean and as pure as possible. In both soil and hydro applications, you want to remove the junk in our tap water as much as possible. Spring water or well water, while “natural” and healthy for us, contains more dissolved solids than tap water. Tap water contains sodium, chlorine, fluoride, and other things that can interfere with nutrients, sometimes locking out essential nutrients by binding with them and not allowing root absorption. Clean water is essential.
Use a Small Boy or an RO system to clean your water. A Reverse Osmosis machine removes practically everything from the water, rendering it with a ppm reading around zero. The drawbacks to an RO machine is that it is very slow, requiring hours to get a usable quantity, and about 1/3 or more of the water becomes unusable waste water which must be discarded. Basically, an RO machine runs water through two filters, and then forces the H2O molecules through a membrane that only allows molecules of that size to pass through. The drawbacks are the slowness, the waste, and the expense of replacing a membrane, as well as the initial cost of the machine itself.
A Small Boy cleans with two filters, removing chlorine and other solids fairly well. The ppm will not measure at zero, but should be around 125 or so. It is fast (1 gallon per minute) and has no wastewater. I have found that this level of cleaning is sufficient for both soil and hydro applications. The machine is reasonably priced, and runs 3,000 gallons before a filter change is needed. The filters are inexpensive. A new liquid product you just add to your water will remove chlorine and clorimides, as well as heavy metals.
Water temperature is vital. Tap water, even after being run through a cleaning machine, is usually around 55 degrees. The water must be warmed to room temperature, either by letting it sit out or by adding cleaned hot water until a temperature around 70 degrees is reached. Roots react very badly to cold temperatures, often shutting down until a proper temperature is reached. Too warm, and you run the risk of rotting your plants. This will interrupt your grow, extending the time and reducing the harvest size and quality.