I've done this several times (not all the time!), and never seen fish stress - not even my GSP's, Xenia's, or shrooms
seem to care (they stay out and extended).
When I have an "emergency" change to do (like when discovered that I lost a fish awhile back), it does usually sit for at least an hour, while I get everything else ready, drain the old water, dump it, etc. I'm only changing 18G out of 90G, so this is not a major PWC
, either. Under 20%. For anything bigger (like 2 batches of 18G) I'd aerate it will overnight for sure.
I've checked pH on replacement water 10 minutes after mixing, and its been dead-on at 8.2.
I know oxygen levels are going to be low or almost non-existent, but by pumping into the sump at a low rate (takes about 45 minutes), that water is going to be slowly mixed with my existing tank water, which is well aerated with 3 powerheads breaking up the surface pretty well, plus the return's turbulence on the surface, plus a sump with lots of turbulence.
It could depend on the salt being used too. I use "Instant Ocean" which seems to mix fast. 10 minutes after a vigorous hand-mixing, the water's clear, the salinity stabilizes and I see no further changes, even after sitting for some time. I always wait for at least the water to clear.
I have a digital kitchen thermometer I use at the tapand I've gotten quite good at reading the temp of tap water with my hands - within one degree usually.
Either way, I've never had fish, or even soft corals/polyps, stress from a PWC
when the water was mixed an hour before adding to the tank.
For top-offs, I usually don't bother to aerate at all, unless I make a big batch of it - the tap water conditioner sets the pH right on most of the time. After alll, I'm only adding one gallon during top-offs to a 95 gallon system - I don't see how its possible that I'm going to affect oxygen or pH levels at all.
I do try to mix ahead of time whenever possible though!