Most dechlorinating products (like Hagen's Aqua Plus) remove both Chlorine and Chloramine in seconds. It is so inexpensive that it isn't really worth it to try and de-chlorinate naturally.
I your water supplier uses Chloramine, I don't believe it can be removed using time and aeration. (I could be wrong on that).
Either way, I believe you have an Arowana. That's an expensive and sensitive fish... so even if you can get all the Chlorine out without using chemical... you still have the issue of water temperature. Unless your heating the water to the same temperature as that which is in your tank... you run the risk of stressing the fish out as a result of fluctuating temperatures.
I have an Aro, Stingray and discus... all sensitive fish. My method is to use Aqua Plus
http://www.hagen.com/usa/aquatic/product.cfm?CAT=1&SUBCAT=121&PROD_ID=01076550020101 in conjuction with water from the tap, running at the same temperature as my tank water. I usually run the water into 5 gal pails at 1 or two degrees higher than my tank water because it usually takes me 20 minutes or so to vacuum the gravel. This way the Aqua Plus has enough time to do its job and the water in the pail will cool a degree or two during that time.
Using this method, my tank temperature usually doesn't vary more than 2 or 3 tenths of a degree while changing 40 gallons of water out of a 180 gallon tank.
So, that's my suggestion. Water conditioner is cheap... just make sure the brand you choose handles Chloramine as well as Chlorine.
Good luck.