Hi Cyn,
I do not have a DSB, but the following is based upon what I have read here.
"Crushed shells" typically means to me chunks of broken up shells (calcium carbonate). The chunks are much larger than what people usually get in "crushed coral", which is crushed coral skeleton (also calcium carbonate).
Sand is much finer than crushed coral (0.1-2 mm diameter). Because it is finer, it creates anerobic conditions better than crushed coral (less pore space and permeability = less circulation potential = less O2). Crushed coral could do the same for you, but you would need a lot of it (i.e. a much deeper bed than the that required with sand).
Nitrate reduction is great with a DSB. They are typically at least 4 in deep for sand. If you want to know how much sand to get for a certain depth, use this equation
tank length * tank width * desired DSB thickness * 0.06 = lbs of sand needed
IMO, if you have a mixture of sand and crushed shells in roughly equal proportions, I would leave it alone. If you have mostly shells, and just a little sand, I would add more sand. IMO, the idea is to fill up the gaps between the shells with the sand, thus creating anerobic conditions for nitrate processing.
You say you have bubbles forming already. This as you know is good, and means your system is working. Congrats.
SOrry it is long,
sam
