I was in question about this set up and actually had plan to change. I was not sure if we had a quake if the rock on the top laying flat would hold, and smash the frount of the tank. Well it took a 5.8 quake and all is well
You should make yourself a certificate of "earthquake surviving rockwork" and stick it to the tank, lol. Putting them flat might look more natural, but whichever way you like it will do.