One effective way to maintain the slope is to use buttresses and/or retaining walls made from slate or other tank-safe stone.
I have 50-60 pounds of Tahitian Moon sand as a substrate, which holds a slope for two or three days at best. The main non-plant features are two large pieces of knarly stump-type driftwood on each side of the tank.
I used thick pieces of black slate to form a stepped retaining wall in the back, between both pices of driftwood. I also angled a thinner piece on the right side of the tank to hold the right-side slope in place. The sloped piece is completely hidden from view and does a great job of keeping the sand in place. The steeped wall in the back is partly exposed, reminiscent of the step pattern of an open-pit mine. As the plants grow in, however, very little of it will remain visible.
As a bonus, my larger piece of driftwood (left side) is basically a large root with an “arm” that butts up against the front glass, forming a “substrate dam.” The substrate behind the dam (front left 1/3 of tank) is about 3.5 inches deep. The substrate on the right side of the dam (front center & right 2/3 of tank) is about one inch deep, creating a nice, natural-looking drop-off effect. Most of the arm is hidden, except for the very top and the edge that forms the drop-off. A rivulet of substrate is able to flow through a small space on the glass side, softening and enhancing the natural looking-appearance.
The chief benefit is that the plants in the back are raised closer to the light, and into the zone where light penetration is strongest. This is important because I have an Eclipse 3 hood with the lights way up front. I am also noticing that debris is collecting at the base of the dam, makijng vacuuming easy.
As my plants continue to grow in after my most recent pruning – which is when I made these changes -- it should hopefully look quite interesting.
29 Gallon Planted
2 Koi Angels -- Mated Pair
3 Rummynose Tetras
20 Neon Tetras
5 Black Phantom Tetras
4 Peppered Cories
Crypt Wendtii Green
Crypt Wendtii Bronze
Crypt Wendtii Red?
Red Ludwigia (repens?)