The really fresh water is chemically "hot" and not a great addition to the tank... It should cure before use if at all possible.

I personally mix water as soon as I can after a water change, and keep it on hand. It saved my bacon twice when I had major tank failures. I don't test the water, but I can't figure out what would cause nitrates... There aren't (should not be) any ammonia sources (organics), and there shouldn't be any nitrifying bacteria.

Sometimes if you keep your saltwater heated, it will precipitate and you will see chalk all over the sides of the container. I've always still used it, and try to remember to keep the heater off if I keep SW for extended periods of time.

Apparently some salts contain a little Nitrates in them or even if you are just using tap water...

but if your using RO I Don't think anything will happen to it....I leave my made up water for a week or so before I add it to the tank and I have never seen any effects from it sitting their!!

I've gone through this before on this forum :) I have no idea "where" the nitrates are coming from, they just seem to pop up. Its fresh RO/DI.. I've not tested the new salt I am using to see if it happens there, but it was a given when I used Kent, that after a few days, nitrates would be up to 5.. and before the end of the week I could count on them being around 20.

As for "chemically Hot" ... never had a problem with immediately mixed water.. so, I'm not about to change.. but to each their own :) Most people say to let it sit 24 hours so it can mix well, but I use a drill driven paint mixer that takes a 45 gallon bucket and turns it into a whirlpool in no time.. so, mixing is not an issue :)