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I've seen people use a layer of the black soil on the bottom with gravel on top of that and it works well. The soil helps with nutrients and the gravel gives them enough space for the roots to grow. My planted tank just has the black soil on the bottom and I think it looks great but I'm thinking of adding some gravel to at least one portion of the tank for contrast and to hide stray roots.
Pool filter sand is great for cichlid tanks and tanks with bottom feeders that like to sift through the substrate. I've found in planted tanks it can smother the roots though. It's too compact and heavy and doesn't give the roots room to grow. Might not be as much of an issue if you have a layer of soil underneath it though.