I know there's the great sand vs gravel debate and most people on this forum seem to prefer sand. Can't blame them it's cheap and it looks great. But one thing I have heard is that gravel can be more conducive to root structure. Also since it's so coarse water can flow through it easily. You don't have to stir it either to prevent gas pockets. Yes I know gas isn't a bad thing but it can be annoying. And some fish and snails will stir it up for you. And it's way easier to clean. The annoyances of cleaning gravel seem to be solved by not having it on top, while below it can aerate a bit and give some extra room for roots. Then sand top better for most fish and easier to clean plus you get the nice smooth looks. So shouldn't a tank with gravel base capped with sand be superior? You get the best of both worlds without a lot of the drawbacks.