This 90 gallon background was built using a horizontal stacking method. The styrofoam pieces on the bottom are intentionally flush with the sides and back of the aquarium to create a type of "shelf" that's higher than the substrate in front of the background. The idea is to help discourage substrate from accumulating behind the rock formations.
This lower left corner section of the background was assembled outside the aquarium for ease of applying the cement on all sides. In this photo, the first of three coats of cement has been applied. This piece includes three terraced "plant pockets" and allows room behind for fish navigation and aquarium equipment placement.
This photo shows how the corner has been cut away to allow room for aquarium equipment placement behind the background. The cave and spaces between the rock layers allow water circulation and room for fish to swim and hide.

The photo also shows how the bottom styrofoam piece was designed to fill the corner of the aquarium to discourage substrate from accumulating behind the background.
This is a rear view of the right corner of the background which is also cut out to allow space for aquarium equipment. A cave like opening (out of view) and large spaces between rocks allow for good water circulation just like on the left corner piece.

The second and third coats of cement have been applied. The final coat included charcoal liquid color.
Generous amounts of GE brand Silicone #1 were appiled to all surfaces where styrofoam or cement met glass. Even with three coats of cement, these pieces would easily float if not secured in place by the silicone.
To reduce the potential for waste accumulating behind the background, an old turkey baster full of cement mix was used to fill in the gaps between the base of the background and the glass.
The area was then smoothed over for a more fish friendly surface. The lower left corner is shown.
The same process with the turkey baster was used in the lower right corner to fill in gaps with cement then smooth by hand.
This corner view shows placement of a hidden CO2 reactor. The reactor is a special powerhead that mixes CO2 and water in a "vortex" chamber then returns to the water column.
Water mixed with CO2 is returned to the water column thru this opening in the rock which will be hidden from view by the shadow of the rock ledge above.
We sometimes see in nature layered rock projecting from the bank of a bend in a winding river. The river, with both time and persistence, has washed away the soft topsoil and exposed the linear patterns of the rock.

This DIY aquarium background attempts to recreate this natural rockscape while providing a fish and plant friendly setting.
For step-by-step instructions on how to build your own custom aquarium background, click here.