I agree with the others totally but have some stuff to add:
For the walls I've found that Astrea snails do a good job of cleaning the filmy buildup in between the times you can get in there with a scraper or scrubby (more of a problem with acrylic than with glass). Also, Nassarius snails are good sand sifters and will help keep the sand somewhat clean, though I usually siphon my sand out every once in a while.
Hope that helps.