Crushed coral traps detritus between the grains and that breaks down to nitrate. It does not trap nitrates in it's pores. LS
is also porous, but is much small grains and packs together resisting detritus settling between it's grains.
Wet/drys do not trap nitrates, they produce them, nitrate is the end product of nitrification, wet/drys do not have an anoxic are for denitrification, which is why they are nitrate factories. LR
and sand, is much less efficient at nitrification than a wet/dry, but they do have anoxic areas for denitrification, because these areas are in proximity to the nitrate producing aerobic areas, they actually reduce nitrates rather than contribute to the problem.