sand or gravel doesn't matter. It depends on how you want to decorate your tank. However not advisable to put driftwood in arowana tank as the arowana might injure itself while playing cat and mouse with your flowerhorn. If your tank is new, I suggest you stabilize the water condition first. Use a few cheap guppies or platies to test the water condition for a few days. What kind of filtration system will you be using? internal or external canister type or sump?