Auto top off replaces evaporated water with new fresh water until your next water change. I'd not use the gravel from your FW tank. I'd only use rocks from the beach if they were cleaned throughly. For salt, I'd stick to Instant Ocean for a FO or FOWLR tank. If you decide to go reef there are others to use instead. I wouldn't call any of them cheap though.
For more ideas you may want to review this thread: http://www.aquariacentral.com/forums/showthread.php?t=118995&highlight=dirty
For more ideas you may want to review this thread: http://www.aquariacentral.com/forums/showthread.php?t=118995&highlight=dirty
