There are a few things every aquarist should know about cleaning the outside glass on their tanks to ensure no contaminants enter your pet fishs' environment. Here's a few basic rules:

:star: If at all possible, refrain from using chemical cleaners. NEVER EVER spray a chemical cleaner such as Windex directly on the aquarium, as overspray will contaminate the water with toxic chemicals.
:star: Preferrably, use a glass cleaning towel. These do not require chemical cleaners or water to clean glass.
:star: You can also use water or vinegar on a towel to clean the glass, if you dry it with a second towel, it will leave a streak-free shine.
:star: If there is a stain that requires that you must use a chemical cleaner, spray the cleaner onto a towel away from the tank, then use just the towel to clean the glass. Afterward, wipe glass again using water solution to take chemical cleaner off of the glass.