I bought one of those little vacuum tubing things at the LFS. Once a week, I vacuum out about 10% of the water into a bucket. I mix my new saltwater in another bucket and slowly scoop it into the aquarium. I hold a plate just below the surface and pour the water onto it. That keeps the water from stirring everything up as much. (I've learned that my water is on the acidic side, so I add a small amount of buffer to the water when I'm mixing in the salt.) I use distilled water from Walmart.
Hope this helps! :happy: :happyfish