Yeah, associated with nitrate problems.
You can use crushed coral, but IMO it MUST be siphoned. Crushed coral has a larger particle size, so fish waste, excess food, etc alll sinks down between the space and can't be reached. So that accumulated waste breaks down into nitrate.
The additional benefit of sand, if it is a deep sand bed, is that if forms an anaerobic layer. This actually helps reduce nitrate levels (because of the bacteria that grow there and use nitrate).
Personally, I think sand in general looks better as well.