I have a 90g FW tank built into an area in the entryway of my house. I'm running an FX5 so it's very simple for me to swap a hose and drain out water. For FW I ran a water line into the cabinet above that I can just pull down for top offs or refills. When I change this to SW here shortly, I can still use this for topoffs, but I haven't figured out how I want to go about handling the refills of SW for water .changes.
This tank will NOT have a sump, so I don't have ready access to add water without going trhrough the top of the tank. I'm looking for suggestions for how to go about doing 10% - 20% water changes.
I can't leave a SW container in the area for a couple days to mix and acclimate because the wife would kill me.

Lunking 10-20g of water and manually dumping it will inevitably lead to messes and much less frequent water changes.
I'm sure there are people here who have tanks in high profile areas in the house that aren't fully plumbed. Can you tell me how you go about filling your tanks for water changes?
How long should SW mix prior to being introduced to your tank?
Can mixed but unused SW be saved for the next water change?
Thanks.