I believe that discus biotopes don't have too many plants, so a thick layer (all over the place) is not necessary.
I'd recommend sand (used in swimming-pool-filters), not a too thick layer. The substrate has to be touched regularly (either by fish (cory's) or by snails), so that it will receive oxygen, to prevent it from rotting. Dirt will stay 'on' the sand instead of sinking between gravel.
Something like this setup example is what I mean: http://www.youtube.com/watch?v=z7DSD9yUA50
I'd recommend sand (used in swimming-pool-filters), not a too thick layer. The substrate has to be touched regularly (either by fish (cory's) or by snails), so that it will receive oxygen, to prevent it from rotting. Dirt will stay 'on' the sand instead of sinking between gravel.
Something like this setup example is what I mean: http://www.youtube.com/watch?v=z7DSD9yUA50