liquid, my biases go to the non-deep sandbed side. There are many reasons (one of the explanations as to why those threads are so obscenely long), but in general: Sandbeds act as a detrital sink. On coral reefs phosphorous is the limiting nutrient in the water column. With concentrations averaging usually less than 1 umol liter-1, most of the nutrients that fuel reef productivity come from benthic recycling.
In our closed system, this is can become overwhelming. Dilution/relocation is a big solution in natural systems. By trying to replicate benthic processing activity via a sandbed in closed systems, we can massively skew levels of bacteria/organics/sinked detritus, concentrating what even on a natural reef would not be broken down on/in/by the reef. The sandbeds are claimed (by some, not to make a blanket statement) to be a natural alternative to removing the stuff via skimming/siphoning/whatever, but in reality, they are just concentrating it to way beyond anything natural. Something can't go to nothing, and it isn't removed by skimming or detritovores or N2 offgassing by a longshot. Hence all the detritus that builds up on a geologic scale in estuarine and abyssal environments.