I'm new to the sw
world, but here's how I've got mine set up. I have three 10 gallon tanks in a closet behind my tank. One is full of pure RO
and plumbed in to my ATO. Both of the others have a line marked at exactly the same spot. One is to fill a few days before I do a water change to allow the water to get up to temp and mix well. The other is so that I know exactly how much to take out, so that I have exactly the same to refill. I have a mini-jet 606 that I use to pump from the 10g to the sump, where I'll be doing my water changes. It isn't a high GPH
pump, but it'll drain a 10g tank in a couple minutes.
Again, haven't actually had a chance to practice this yet, as I've yet to do a water change (cycling), but it worked well for filling the tank.