I read in a book that you should stack it from front to the back and water should be able to pass through it. It my 20 gal long. I order 20 pounds and got 3 big rocks, used two as a base, then stacked the third on top of the two, to make a cave. Ordered 12 more lbs and used these on the two sides of the tank to make more caves, plus a place in the back to get away from people if stressed. you don't want a brick wall type of stacking.
That just want I know, anyone esle?