To be honest, I've never setup a dirted tank before. I've just seen others posting about it. I believe gravel would have trouble containing it with the larger gaps in it. Sand would form a compacted "cap"and contain the organic soil better. I think you can try pool filter sand as it's inert and pretty inexpensive. Someone correct me if I'm wrong and chime in please... Otherwise, google it and I'm sure you'll dig up some threads on it.