As substrate for this tank I used pool filter sand, which I find to be an almost ideal aquarium substrate for the following reasons:

Perfect grain size

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 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, 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.

Chemically inert

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 like my 125G tropheus tank 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 even in the mix. I used one bag (15 pounds) of Carib Sea aragonite for my 125G tank.

Inexpensive

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.

Readily available

Pool filter sand is available for wherever you can buy pool supplies. In my area that's the Litehouse Store Chain. In those States of the US that suffer a cold winter during which outdoor pools can not be used, pool filter sand can be hard to come by during that season, but you can stock up in Fall, or you just have to wait until Spring.

Clean

People wanting to use this stuff to filter their pool don't want to clean it before use, and they don't want it to cloud their pool either. That's why pool filter sand tends to be very clean right out of the bag. I didn't have to wash mine at all, while for example play sand can be a pest to clean.

The catch

There is only one catch, and that's that pool filter sand is only available in one color. If you want dark substrate, have a look at 3M Color Quartz. Apart from being available in a wide range of different colors, it shares the main advantages of pool filter sand in that it has a very similar grain size (see photo above), and is chemically inert. However, it is a lot more expensive (about $20 per 50 pound bag), it is hard to come by in some areas of the US, and it needs a good washing before it can go in a tank!