I remember once I had a tank with sand, and some of the fish were totally different with. I had a horseface loach that would just dive right under and "swim" through the sand and come up somewhere else. Also had some small horseshoe crabs that would bury themselves in it - never had much luck with them when I had gravel, but they were pretty hardy with the sand. The only problem I had was when I tried to setup this small eco system. I had a 30 gallon tall tank, kept it about 1/3 full of water with sand, a couple fish, had a small turtle, a couple tree frogs, and yes, you read this right, I had a couple zebra finches. The birds would sit on the branches that crossed over the water. It actually was really cool. But the problem was the bird food (seeds) would fall into the water, and start to decay under the sand, and the smell was terrible. I tried to find fish that would eat seeds but it did work out. But for a few months, had a really cool looking tank.