You can put it over the coarser stuff but it will eventually settle down and underneath it.
Depending on what kind of sand you're talking about, you might also have to be a bit cautious in doing it so you don't create a sandstorm.
Though, if all this tank has is whats in your sig line then thats not too much of a problem, it just might be dusty afterwards and require a little cleaning afterwards.
I just added a section of southdown sand to my otherwise barebottom 72.. - I put it in a large mixing bowl, put plastic wrap (loosely) over the top of it and let it sit on the bottom of the tank for a couple weeks before slowly pouring it out of the mixing bowl. - The extra time gave bacteria a chance to colonize and weigh down the dusty grains and keep it from getting blown all over the tank when I poured it out. -- Oh, yeah, and I also mixed the sand up by hand every couple of days too to allow bacteria to colonize several layers and not just the upper surface...
Good luck with it!
Norm
