Bringing tapwater to life
We have an intuitive feeling that tapwater is lifeless. There must be a "something" we can add to tapwater to help "bring it to life" in the aquarium. That magic "something" could be a teaspoon of salt, or a teaspoon of a "conditioner." These things are more concrete than a prayer or incantation, but sometimes not less magical. The water that comes out of your tap is pretty much lifeless, it's true. Your local water company works hard to bring you water that is as lifeless as can be achieved, and you and your neighbors wouldn't have it any other way!
In chlorination, elemental chlorine gas (Cl2) has been replaced nowadays by volatile liquid sodium hypochlorite (NaO2Cl2). Most commonly it's added at the pumping station at the last practical moment, because sodium hypochlorite remains dissolved in the water supply only as long as the water is under pressure. As soon as the tapwater is released from the faucet, the chlorine begins to outgas. You smell that released gaseous chlorine when it comes out of your tap under pressure, simply because the chlorine is rapidly diffusing into the air, once the pressure is released. One of the reasons utilities are switching to chloramines is that they are more stable than sodium hypochlorite and don't simply outgas. That's good for your neighborhood, bad for your fishes. Chlorine/chloramines get their own page here.
There's no mystery about the gases dissolved in your tapwater: they're simply the gases of the atmosphere, kept under pressure until the water is released from the tap. Then you may see a zillion tiny bubbles turning the tapwater cloudy for a few moments and collecting on the sides of a glass. At this moment they would be collecting on the glass and plants and rocks in the aquarium, and even on the fishes' gills. By the way, even if your aquarium is densely planted and intensely lit, this is not the phenomenon known as plants "pearling."
Part of your job as an aquarist is to bring this artificially sterilized water to life in the aquarium. And your first job is to prepare it so that it won't kill the bacteria in the aquarium by dealing with the chlorine/chloramine. As long as you don't have chloramines in your water, you can simply let the chlorine outgas naturally for 24 hours. Once it is added to the aquarium, the bacterial and planktonic life in your aquarium water will take it over and "remake" it.
Gases and the other subtances dissolved in water are the subjects of the next three folders in the menu at left, and the pages within them.