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