I know the "at least 1 or 1 1/2 pounds per gallon" rule, but I got mine from a guy taking down his tank and didn't really know how many pounds it was. It was in a five gallon bucket about half way full with rock. How much space should the rock take up in the aquarium? I have a 24 gallon nano cube. I put the rock mostly along the back wall stacked up about a third of the way to the top, but most of the aquarium is open swimming room. However, when I look at pictures of other people's nano cubes, some seem as if the tank is almost full of rock from back to front with hardly no swimming room.