Refers to a diatomaceous earth filter, which uses sedimentary rock as the filter medium. To use this kind of filter, the D.E. needs to be added when the pump is on, using a skimmer; this takes the D.E. and deposits it on a grid, which leads to the substance becoming the filter medium.
It was decided that a D.E. filter would be used, as it was the most compact and did a better job of filtering out smaller particles than other filter types.