Def consider sand for the Bichir. Thier eyesight isnt great, so there is a high possibility that when they are eating, they may goble up a rock in the excitement. Theres a big chance that if they do, they will not live. (I know some do, but a small ones chances are very slim). Plus, because they are mainly bottom dwellers, sand is better because its small and usualy smooth (depending on what brand/grain size). This way there is less chances of them getting cut when they are swimming around the bottom.