the substrate is somewhat of an organism of it's own, haha. it was just plain black gravel until this past winter, when i realized a higher pH would work better for the gars...so i added African Cichlid sand (which is aragonite -like in saltwater substrate- and some black sand), and then a ton more of regular white African Cichlid sand (primarily aragonite).
this way the substrate buffers the water better, and allows for minimal pH fluctuations in between water changes...sensitive fishes, like the Cuban gars do very well with this setup.--
--solomon