Reason why I'm asking, is that I'm seeing a reddish tint to the water, and for this weekend's water change, I want to do a change, refill the tank, and then do a second change right afterwards. I don't want to pour in new water, and then take out the new water only. I'd like to take out the old water as well.

Now does the water mix in immediately, or does it take a few hours to mix in? I have a tank that is 25-30 gallons, with an Aquaclear 50 filter.