I do a weekly 20% water exchange and what I would like to know is, after you have removed the 5 gallons, how fast should you add the new water back in? The new water is at the same temp. as the water in the tank.
Adding water-change water back in shouldn't be a big issue if you add it fast since the salinity should be the same. With top-off water, I try to pour it in a high flow area to distribute the water quicker.
You should also try to match the waters pH as well as the salinity and temp. Then just add it back as slow as necessary to prevent stirring up your substrate.
I know it's probably over kill, but I siphon the old out and siphon the new in. It's slow but it's easy and the occupants of the tank hardly even notice it. The hard part is getting the bucket above the fish tank to siphon it back in.
