Sand over pond soil in my 30 gallon, small grade gravel in my 5 gallon Hex, and small grade gravel in my 12 gallon, but I am replacing this with black sand as soon as I have hoarded enough bags of it ( I think 3 bags in total should do it )
It's actually reptile sand which is both dye and chemical free, so it's safe for fish but rather expensive at 7.50 a bag!!!! I think it'll look good though, and will show up my pygmy cories ,endlers, black and cream guppies and Golden apple snail very well indeed.