debris or detritus?

The purpose of an undergravel filter is to be under a substrate. This is where the beneficial bacteria will reside to break down food. With no substrate, you would just have an ugly tank bottom with no purpose IMO.

 

I've never tried a low nutrient system, so I can't say if feeding is unnecessary simply because of pods. Corals generally benefit from the presence of fish, as nearly all will absorb dissolved organics from the water and many also feed on particulate matter. Those that feed on particulates will most certainly eat live pods and zooplankton, but I'd be reluctant to say if that would suffice long-term.

BTW, pods mostly feed on algae and/or phytoplankton, and their population is controlled by available food.