I find Pool filter sand to be an almost ideal aquarium substrate for the following reasons:
Perfect grain size
Pool filter sand is coarser than play sand but still a lot finer than any aquarium gravel. Play sand is very fine, which unfortunately means it tends to clump together. Clumping can lead to anaerobic pockets in the substrate, which can produce chemicals that are harmful to fish and have a bad odor. With aquarium gravel the crevices between the particles are large enough to trap leftover food and other debris, which need to be removed by gravel vacuuming. Pool filter sand isn't so fine that it clumps, but fine enough that debris usually stays on top of it from where it gets swept into the filters or is easier to remove by vacuuming.
From left to right; top row: play sand, pool filter sand, black T-grade 3M Color Quartz; bottom row: fine natural-looking aquarium gravel, fine black&white aquarium gravel. Each container measures about 1.5" in diameter.
Pool filter sand is completely inert, meaning it will not affect your water chemistry. This makes it suitable for setups with soft water and hard water alike. For a Rift Lake Setup it is desirable to buffer the pH and slightly raise hardness. This can be done easily by mixing some aragonite (crushed coral) in with the pool filter sand. The color and grain size of these substrates is very similar, producing a nice even look in the mix. I did this for a 125G Tropheus setup, and used one bag (15 pounds) of Carib Sea aragonite for the tank.
Aragonite (crushed coral) can be mixed with Pool Filter Sand for Malawi and Tanganyika setups.
I paid $22 for 15 pounds of Carib Sea aragonite, and did not consider that a bad price. A 50 pound bag of pool filter sand is usually retails under $5, which makes is almost as cheap as play sand.