a topic that everyone with an aquarium will deal with.
I have a 100Gal food grade vertical plastic container in my furnace room, RO/DI unit in the same room. It will take a full 20 hours to fill the tank, I have an 400 GPH pump submerged to the bottom and leave it running all the time.
I add approx 9 full cups from my trust salt measuring dispenser, let it sit and mix for a day and verify salinity. I do use a heater since the ambient temp of my furnace room is a few degrees lower than the rest of my basement.
I have 30 feet of 0.5 inch tubing I hook into another submersible pump for water changes, after siphoning out about 80 Gal. I due 80 to 95 Gal every two weeks, but I have over 300Gal of total water volume.
Since I have limited space in my furnace room I went with a 23" wide but 65" tall container. It was just dump luck that the power cords attached to my heater, and pumps just pops out the top of the removable lid. I forgot about that part when I got the tall tank, thinking I was all smart n stuff
but...it worked in the end.