Some water departments also add lime to raise the pH of the water and limit dissolving of the lead in the pipes. Somehow, a little aluminum sometimes may also get into the mix. (People in modern industrial countries have 1000 times the lead content in their systems that a medieval European would have - according to analysis of hair samples.)
Reverse Osmosis (R.O.) sellers have been advertizing their units (which take most everything dissolved in the water out) to produce water for mixing baby formula in. Evidentally (and horrifyingly) the ammonia (= nitrates) in water has cost at least one infant it's life.
The so called growth inhibiting substances released by fish in aquarium water may be in part simply the nitrogenous waste they and all living animals excrete. Excess nitrogen (I am told) decreases the efficient use of oxygen and bodily processes (growth, disease prevention, healing, reproduction....) This means that we need to let our water season.
Seasoned water has meant different things to different people. Often it only means a day or two wait. For some of us more involved in the hobby, it means filling up a large container, letting it sit several days and then drawing water from it over the next week or three. In one case, an individual filled up a cistern and aged water could even be a year old!
Probably a week, more or less, is a pretty good rule of thumb. Every day, the water will release a little chlorine. There will still be measurable amounts of chlorine at week's end - but it will be considerably diminished from the start of the week. That can be accelerated with a water conditioner or even aeration.
There was a time when American aquarists could turn on our water taps, adjust the warm and cold water and pretty much refill the tanks. One wouldn't want to try that in most places today.
(There are a few lucky ducks still...)
I can't believe that (as a kid) I got away with dumping my bowl of gold fish in one of the kitchen sinks (it was the rinsing side), scrubbing out the bowl with a paper towel and refilling that bowl with tap water (measured by feel to equalize temperatures) and dropping the goldfish back in. If it wasn't for the cat, they might have lived forever. ;)
If you don't know what is in the water or what had been added, check with your town's water department. They should have an EPA analysis sheet of what's in there. Again your tax dollars at work. And it is your right to know.
I have used gallon water jugs, an extra aquarium, camp ground carboys, those five gallon water jugs from water coolers, (seasoned) plastic garbage cans a couple of years old (used for garden water, bleaching dirty tanks, etc.) to store water.
People have stored water in stock watering tanks. They have utilized buckets and barrels formerly used to store food in - after they were bleached, rinsed with hot water, and aired in the sun. Sometimes the larger containers are set on styrofoam (old fish boxes never die) to insulate them against the floor or blocks they stand on. Good submersible heaters have been used to raise their temperature a degree or two above room temperature.
Probably most any soapless, greaseless, food quality plastic could be used. Maybe stainless steel - avoid other metals - could be used in a pinch. Old ceramic pickle crocks (those are probably collector's items now) and other voluminous food holders could work. I've often wondered about those 20 gallon all purpose plastic tubs at the big box hardware stores.
There are aquarists who have set up a large "vat" on cinder blocks so that they can change water just with the use of gravity and a siphon. It gets even more complicated if a person is keeping not only tap water, but an RO mix for spawning rainforest fish and or a mix of brackish water. (I try to keep 30 gallons of our "liquid rock" tap water and 50 gallons of R.O. water on hand at all times. No conditioners are used. Sometimes heaters and air stones are.)
Be glad that guppies are generally comfortable with most temperate (North American, Turkish and Paraguayan) tap waters. People in tropical regions and western Europe may have to be more careful. (The radical water shift may be why guppies raised in Singapore don't always fair too well when imported elsewhere if they haven't been properly acclimated to the new water.)
Although there are a few Poeciliids from waters with low mineral counts, for the most part livebearers in the hobby are hardwater fish and pretty easy to keep in local water... if - of course - it is fairly frequently changed.
(For a further disussion of this see the thread on "flow through systems"
http://fins.actwin.com/killietalk/month.200310/index.html )
Here's my question to you. What do you season your aquarium water in? We all promise not to tell the rest of your (and my) families what we are up to. ;)
Thanks