*nod*
I've also read for the base to just use standard base rock... for a 12-gal. tank, however, there isn't much of a footprint, anyway... so to speak... I suppose more importantly, regardless of weight... it also depends highly on the size of the pieces and the look you are going for. For an initial order as mentioned previously, I'd say you'd wanna be somewhere between 8lbs - 12lbs.