Originally Posted by sharonar22
I don't know much about sand either, sorry, although I just called my LFS
about all this brown stuff on it and they said to 'stir it up'
That wasn't good advice from them at all. Unless the sand bed is super duper thin, you really don't want to mess with the layers. Vacuuming the top 1/2" or so is fine every once in a while, but "stirring it up" will just release all the nasties back into your tank before they've had a chance to fully break down. That could then lead to another cycle, with spikes in ammonia and nitrites. Granted... I think your tank is new, so there's not much to stir up, but down the road I wouldn't recommend it.
I'm guessing the brown stuff - if it's like rust - are just diatoms, which are normal for new tanks. They'll eventually burn themselves out. If your cycle is complete, then adding a good clean up crew of astrea or trochus snails would be another way of getting rid of them. Trochus snails will devour the stuff in no time.
If the brown stuff is slimy and sheet-like, then it's most likely cyanobacteria which normally is from too many nutrients - either from overfeeding, lack of water changes, or using tap water with high nitrates or phosphates in it. Better water quality, as well as siphoning out the cyano
along with a little sand, is a good first order of defense against the stuff. If you continue having issues, then do some searching here on "cyanobacteria" and you'll find a lot of good advice!