When it comes to cleaning aquarium glass, you need to make sure you're cleaning the surfaces in a way that is safe for the fish, but will get the job done. Also, you want to minimize the chance that you will scratch the aquarium glass while cleaning.
You can clean the outside aquarium glass with any conventional glass cleaner but be careful: getting any in the tank can be harmful to your fish. If you are using a store-bought window cleaner, spray some of it on paper towel, possibly in another room, instead of directly onto the tank.
Even if you choose to clean the outside glass with a homemade cleaner, however, such as vinegar and water, you should still spray it on a paper towel away from the tank. Any foreign liquids in your tank could upset the PH balance and make your fish sick.
When cleaning the inside aquarium glass, never use a cleaning product. This will contaminate your tank and possibly kill your fish.
Instead, when performing a water change, take a clean paper towel and gently wipe off the inside of the tank, making sure not to leave behind any pieces of the towel. For cleaning in between water changes, you can purchase special cleaning magnets at any pet store. One magnet goes on the inside of the tank and is magnetized to the other on the outside. Then, by rubbing the outside magnet around the glass, the inside magnet cleans off algae and waste. Be careful when cleaning near the gravel in the bottom of the tank with this method, however, or you could scratch your aquarium.