I've got just a couple more ideas to add.

I change about 3 gallons/week in my 30 gal tank. But I mix up about 25 gal of salt water all at once in a trash can on my porch, and will use that until it needs replenished. That makes my weekly water changes super easy.

Also for top-off freshwater, if you don't want to just poor it all in at once and potentially shock sensitive corals or whatever, here's one easy+cheap option to slow down the rate freshwater is added:
You can get a cheap bucket (like a big icecream tub)
make a hole in the bottom corner,
put and end of an air-tube into hole and seal it off with silicon,
add knots to the tube to slow down the flow

Now when you need to top off fresh water:
set the bucket ontop or above your tank, filling it with freshwater you need to add, and let it slowly run into your tank.