The smallest objects farthest in the background need to be placed very high and highly visible so the onlooker can still see them well.
This aquarium layouting method makes use of a very simple optical illusion. Basically you can say that large objects appear to be closer and small objects suggest a greater distance. If you place the largest objects in the foreground and continue into the background choosing objects of a constantly decreasing size the sense of depth you create is extremely strong, of course always in combination with the first basic rule of mounding the substrate. The smallest objects farthest in the background need to be placed very high and highly visible so the onlooker can still see them well.