Many topics in Hydroponics are arguable, often debated. Even Ed Rosenthal and Jorge Cervantes disagree on many subjects, LIGHT and WATER being the main two.
Jorge Cervantes says to start with PURE water, R.O. Water, (Reversed Osmosis) or Distilled water, or Filtered Water. Ed Rosenthal says CITY water works just fine, as well as Well Water.
One of the not so simple procedures for many growers is maintaining a level pH. And if the plants are eating and drinking daily, the pH will change daily as they eat and drink, removing nutrients from the water.
I will not get into a bunch of scientific points to make here, I will only report on personal experience. I do accept and believe the more controversial side of the coin, that City Water and Well Water not only works or performs just fine, but works best for me.
If you will google "Water, pH, Buffers" and research it, you will find both sides of the coin. You will learn that water with content, (water with Lime, Calcium, Iron, Maganese, Magnisium, and other minerals) is easier to maintain a level pH, than Filtered Water, R.O. Water, or Distilled Water. ( a buffer is a stabilizer)
Those minerals act as "buffers" holding or maintaining the pH at one level.
Water that has no content will drift up or down in pH more quickly and need additonal added buffers.
Then there is the dreaded "clorine" in the city water.
I once took a job selling Water Filters. I had to go to a short class for two days, studying city water, water filters, and clorine. We would go to a prospect's house, with a clorine swimming pool test kit. We would test thier water and frighten them, and show them it was too highly clorinated to swim in, much less drink it. We'd hook up the filter, let them use it for free for a week, drinking their tea and coffee with no clorine in it. When I came back a week later, they would not want to give up their filter. They would buy it everytime.
We were taught and had it proven to us, that clorine will "disapate" ( a fancy word for vaporize, or disappear in vapors) with any added heat, aeration, or being allowed to sit an hour.
In the past two years, I did two very successful grows at my X-wife's house, and she has city water. We tested it with a swimming pool clorine test kit. It was very high in clorine. I ran the water from a faucet, over the warm palm of my hand, into a large metal bowl. When the bowl filled, I poured it into a five gallon bucket, one bowl at a time, holding the bowl up high, allowing the water to fall through the air. When the bucket was full, we stirred it, tested it, and 75% to 80% of the clorine was gone, no smell, no odor, no taste.
Today most citie use chlorinated water. Many cities have switched to a form called Chloramine. This form will not dissipate, will not evaporate, will stay in the water forever.
You can check if your city uses Chlormine on your water supply's website. All info must be reported on Chloramine levels.
You can find Dechlorinators at your local aquarium stores. Most often it is sold to people with large fresh water home made ponds. Generally they run city water, but do not want the chlorine or chlormine to hurt there VERY pricey Coy or other fish. Dechlorinators work great, and are easy to use. Many growers still report great growing results with city water.
Three years ago, I tried to do a grow with store bought distilled water at a friend's house. His water had a sulphur smell to it. I read it was best to use filtered water, in a Jorge Cervantes book. Cost us 79 cents a gallon then. I only did one 6 gallon tank of water, and after we bought and hauled water for 4 weeks, spent about $30 to $40, and battled and battled pH spikes and pH flucuations for a month, I wised up. It is very difficult to maintain a level pH in Filtered or Distilled Water.
And there is water with energy or electolytes and there is dead water. I want my water full of energy, not dead from filtration. So I use and prefer well water.
