Aging the water before adding to your tank is always a great idea.
It allows the PH to stabilize, dissolved gasses to dissipate and a number of other positives.
http://www.monsterfishkeepers.com/forums/showthread.php?253999-Aged-Water/page2
Couldn't say it much better so here is a quoted
post by DavidW from the thread linked above.
"Anyone using water direct from an urban water supply is playing russian roulette.
All urban water supplies are pressurized and also much colder than your tank. Straight from the tap there will be a surfeit of dissolved CO2 ( and other gasses) and a deficit of dissolved O2, along with a rich dose of chlorine in most cases, chloramine in others and also usually fluoride.
This colder water will go immediately to the bottom of your tank ( convection) and on the way release the CO2. You will often see little bubbles of it on plants or glass or even on the fish's skin. CO2 is also acidic and pH goes up when it gasses out.
Chlorine is unstable in water and will naturally 'gas out' in about 12 hours without any need for conditioners etc.
It would be interesting to watch a dissolved gas meter bouncing around in most of your tanks.
Urban water supplies will occasionally put a 'bullet' of chemicals into the system to manage seasonal microfauna blooms, but they won;t tell you about it.
Adding any water conditioner to the tank directly is imo crazy.
Doing the above means essentially you are putting your ray in the middle of an active chemical reaction involving bounces in temperature, pressure, pH etc.
Even people with Well water are susceptible, athough the chlorine/chloramine issue is not there.
I learned all this the hard way.
I predict that within the next 12 months at least 1 of you here that does not age/condition your water outside the tank will have a major catastrophe.
Folks that use a direct drip constant flow system through prefilters have a much lesser chance of this problem, although not even R/O will remove chloramines if your water company suddenly decides to change its treatment protocols.
If you have chloramines then you have no real choice but to use a conditioner to neutralize it but you really should do this in a tub , even if it is only an hour before you add the water to your tank. I find it contradictory for anyone to recommend mixing simple salt with water before adding it to the tank but no problem to add a noxious chemical like prime etc"
