In case you haven't been told, newer aquariums go through a series of algae phases, which include things like diatoms (good, normal, food for bacteria), cyanobacteria (normal, undesirable, can be frustrating to remove), detritus (normal, made up of fish waste or decaying food), or lastly dinoflagellates (awful, something you'll never want to deal with).
Rinsing out used sand saves money, and you can safely put that clean sand in your next tank without concern. It's a good project to tackle in the warmer months, best done outdoors. Easy, sort of.
More reading here: http://www.reefaddicts.com/entry.php/28952-Rinsing-used-sand-for-the-new-setup