Hey Boonjob Why do you have a 3 day mark. The water should be good for awhile.My water usually sits for a couple of nights before going into the tank. A couple things to consider would be keeping it closed and kept in a dark place.You don't want anything to get into it. Something bad falls in and breaks down into ammonia. Then your just putting dirty water in your tank. I like keeping mine closed to help with evaporation also. If its dark I don't get any algae. Also you need to bring it to the proper temp and aerate the water before you do your change this balances the pH by either releasing or absorbing CO2, and raises the ORP which may have a good effect. Not to mention it is helpful or the fish and corals too.
It really is just something I do personally, but to list the reasons...
All my water is made in the same barrels you bought from me, and stored in sealed containers that are not hit by much to no light at all(stored in a basement). I allow all the water to circulate/aerate/stabilize PH/ and heat up over 24hrs before even considering putting it in any containers or tanks.
Now the reasons I don't allow the water to sit for more than 3 days; The major one is the longer I let it sit the more I am apt to neglect it, so I just keep myself on routine by setting a mark. I also have several people who I make water for, and most of them store their water for 6-7 days, so that leads to me making a fresh batch every 3-5 days(after calculating the time to allow the ro/di to make the water) so I only let it sit for no more than 3 days so that I can clean any cotainers or components that I see fit before the next batch. Also it is minimal, but storing the water for any longer would require more power/electricity, aswell as more hours on my equipment which I really dont see the need for. I am usually spot on with the amounts I make, and anything extra I just do a WC in my tank, however this time around I had over 80 gallons extra, so I used about 55 on my tank last night and have over 25 left.
I completely agree I could stretch it further but I have alot of people that I provide water too as it is and doing it this way has yet to yield any problems. and if it ain't broke I dont fix it ;P