I agree with Jexnell regarding a method to introduce and remove water from the tank. Excellent advice.
So I'll address the saltwater and cleaning, as my freshwater 240 was loaded with hard stains l after being used as a saltwater tank for years.
In order of least-to-most aaggressive.
1) Spray bottle filled with cleaning vinegar. (You can dilute if desired). Spay must the inside glass and let soak for awhile. You can stick some paper towels over the spray to keep the solution on the glass. Rinse with clean water.
2) Same as above, but agitate the solution with a plain Magic Eraser. (Melamine sponge).You can lubricate the sponge with a few drops of Stress coat, but it shouldn't scratch regardless. Rinse.
3) Same as above, but sprinkle Bar Keepers Friend on the sponge. Rinse well. This should take out 98% of all the salt creep/stains.
4) If you need to polish out any stains/ scratches...use cerium oxide powder. Rinse well.. This is as far as I've gone.