And, if you remove a bit, add a bit, remove a bit, etc, you're removing some of the new water you just added. Counter-productive in my mind. Remove the full amount you're changing, THEN add the new stuff.

Agreed. Same method I will be incorporating.

Hmm, I currently don't have a sump & never thought about this little quirk. I generally do fairly large water changes less frequently in my 75 so a lot of gunk and old water gets siphoned out before I add the fresh water manually. This isn't too much of a hassle since I don't have a hood over the DT making it easy to access.

Something to consider when I set up my 90 with basement sump.... make sure I use a large sump/refugium if the plan is to continue with large water changes. Otherwise I'll need to change to smaller more frequent water changes.... or turn the return pump off and continue with the manual changes in the DT on the new system. That will take considerably more effort since I'm planning the new set up in a room dividing configuration with full hood. Good thing I have a spare 65 kicking around, that should make a dandy sump with plenty of extra capacity. Once the 75 is empty, I might even plumb that into the system too...

I do the same. I mix my new SW a couple of nights before and let it sit with a heater so that it is the same temp as the tank.

Got the room divider upstairs with the basement sump but still during a WC I'll siphon out from the display (~60l) into a container that I'll then just wheel into the main flr bathroom to dump. Rest from the fuge and sump I siphon into a laundry sink I put in for the tank.

Thanks for that info, I'll most likely end up with a similar routine... Apologies to the original poster for changing the subject, but 'mark' what are you using as a return pump from the basement, how much head pressure & flow is thru your sump? How high off the floor in the basement is your sump?