Your best bet tom get some benefit from crushed shell id to use something that6 resembles a carbon bag. Instead of carbon, use the crushed shell or crushed coral. Where you want to end up is with a package of calcium carbonate, the chemical composition of crushed shell, in your main filter's flow path. With the CaCo3 in that location, you can expect to have a higher pH and a higher carbonate hardness, KH, than you had before. That means you will need to worry less about having the buffering capacity of your tank water depleted. The end result should be a more stable tank water chemistry and lower ammonia and nitrites than otherwise. In this position, I expect the buffering provided by the Calcium carbonate to prevent my tank water chemistry from wandering.