Our fire bellies both have gravel in their tanks, and we haven't had any trouble with it getting eaten, which I think is due to the way they eat - forward 'jerking' motions, rather than 'hoovering'.
The water gets quite mucky due to the food, but Azhael is right in saying that the gravel traps a lot of dirt, we have to wash the gravel once a fortnight to stop the water getting too bad.
My paddle-tails have a bare-bottom tank, and it is easier to clean. I haven't tried sand yet