Pool filter sand is great, it's sifted so the grain size is relatively uniform and doesn't compact like play sand. If you want a darker colored substrate look around for Black Diamond or Black Beauty blasting grit. It's actually coal slag, no silica so no diatom bloom, and works as well as pool filter sand IME. Also dirt cheap and easier than sand to rinse, just get the largest grain size. I've had it in my 220 for several years now, and have never had gas bubbles or anaerobic pockets even with several inches deep.