I use pool filter sand because it's cheap, easy, and I already had some. Many also use black diamond blasting media, which is a coal byproduct used in sandblasting, you can get it at farm stores.
Either one of these products is inert (no plant nutrients) so they say you need root tabs (fertilizer). I have a 75, 55, &10 gallon tanks and don't use any to grow low light plants.
The pool filter sand is light, where the black diamond is...well...black.
I would not use gravel ever again after using sand, if you want to spend for $ you can get substrate specifically for plants ( nutrients in it).
Here are pics of mine with sand.
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