Never did it during water changes. I'd aerate it for about 6-10 hours, then add salt, then adjust any levels accordingly. But recently read it's a good idea to buffer rodi with baking soda first, I guess to the final alk u want or maybe a little under, then add your salt. Otherwise you'll lose alot of the salt's buffers up front. I've never done this & was wondering if maybe I should've been all along-- been successful with sps for years now but always room for improvement. Anyway, setting up brand new tank, about 140 gallons total, it's full of rodi now & wondering if I should add some baking soda before I start pouring in salt. Thanks!