I think they meant in the tank. what tribe says is true for the outside! Put polish on the cloth dont spray it near the tank.
It depends on your tank set up and how big your tank is really on what you can get. Some "algae eaters" also called otos are good for cleaning glass depending on how big a problem you have. some plecs will eat a limited amount of algae, but you are probably better buying a scraper and spending 5mins scraping the algae from the glass. they arent that expensive and if you have kids can be easy to do as they will do it for you. My neice and nephew are always using my brother-laws scraper thing (the one that uses a magnetised bit in the tank).
Hope that helps,
Craig