it could be trapping detritus, increasing nitrate levels since it will trap the waste and cause more water quality issues not to mention unsightly algae..
Most of us will not use crushed coral because it is a large substrate that traps the fish waste and uneaten food that has to be vacuumed before it creates nitrates, which it will anyway. Crushed coral does not provide a very good biological zone, and many tanks are setup with CC from the get go through lack of knowledge or because it is the only substrate that an LFS sells and tells you that it is all you need, using a selling point of CC has buffering power. So many of us have been there and had high nitrates, did a water change to lower them and they were back in a couple of days. CC has sharp edges, which is undesirable for inverts, like anemones walking around, pods or worms. No getting around it CC is high maintenance and can lead to poor water quality, frequent maintenance, sick livestock, algae blooms and more.
Sand on the other hand has more benefits. These include having far more surface area thereby making it able to handle a higher bio load of bacteria. It is less dangerous to your infauna and has a more natural look in the tank. If going with a DSB Deep Sand Bed you can have other benefits as well like finishing the denitrification or providing sand sifting, burrowing, or tunneling fish and critters a place to play. The denitrification process predominantly occurs in deeper substrates and in areas of stagnant flow where oxygen levels are depressed. And this is why deep sand beds are effective as a nitrogen export mechanism. As water slowly diffuses deeper, aerobic organisms strip all available oxygen for respiration. In the deep, oxygen-deprived layers, denitrifying anaerobes are given the opportunity to convert nitrogen compounds into nitrogenous gases, which escape via tiny bubble out of the aquarium. I believe this process can also work on a limited basis in shallow sand beds.
As to the mixing of live sand and crushed coral. Doing so would be a mistake. First off the CC would rise to the top and the sand would push to the bottom, eventually all you would see is the crushed coral on top. This CC would then still trap detritus and need to be vacuumed, and you would just be sucking up some sand with it every time you did it. The sand bed under the CC might not function correctly or build up anoxic regions and if you broke into those areas with your syphon you could have a crash of the tank. The water flow would not reach the sand correctly sending it the nutrients it needed to process it. Instead the nutrients would fall on top of it through the crushed coral where it would stagnate until you vacuumed it.
The idea with sand and good water flow is that larger particles of detritus would stay in the water column where the mechanical filtration would remove it, and the dissolved particles would flow into the sand bed where the infauna can process it into smaller pieces and the sand bed can process it into a harmless gas that escapes the tank.
