Yeah, I've dumped my extra out too, guess I'm a little anal about it. I usually mix what I need and let it go with a PH in it for a few hours until the salt is completely disolved, then I bring the temp up by tossing in a heater and finish off the Salinity 'fine tuning'.... I usually do the water change within a few hours of this... I've read 'both' that you are supposed to let the salt mix stabilize overnight AND that you can mix and dose the new water immediately... guess that puts me on the fence
That being said, when I setup my current system (~75g) I borrowed a water trailer from my LFS. They premixed it to 1.024 and as I was checking the water before I hauled it to my new home and started setting up the tanks, I realized that it smelled HORRIBLE. Upon closer inspection, it had chunks of algae and crud floating around in it and growing on the inside of the 500g container... Turns out that they had a week old 75 gallons of water and decided to just add new DI water on top and mix it back up to spec. The water had been sitting outside in the sun in a sealed 500g agri drum (large white drums you see on yard crew trucks) with no aeration or circulation for 6 full days... It was quite nasty.
Unfortunatly, I was in a bind and had tubs of coral, LiveRock, LiveSand, buckets of fish/eel sitting in my living room slowly getting colder/dryer... So I "had" to use the water. It was going to take them 2 days to make a new batch...
I setup a temp solution for the babies (my critters) in my living room, with air tubes, heaters, and powerheads running everywhere as I finished moving in. The water trailer was in my garage running through a commercial grade filter for 2 days before it stopped smelling like 'feet' and ammonia.
This is the water I used to setup my system.... and all is alive and well 8 months later (including the cyano, but I don't think that's the fault of the water
)
Needless to say - I got the ~100 gallons of water I used for free and will know better than to rely on that LFS in the future.
(the previous is "NOT" the recommended proceedure for moving your system... just in case
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Best of luck,
John.