Black is the least dense material. We normally use it only as an initial coarse leaf filter. Order Gray for the best Bio Filtration. Stack the colors to allow the water to pass first thru the Green, then Blue and finally thru the Gray sheets. This creates an ever increasing filter density that removes most debris before the water reaches the tightest density pads where most of the Bio filtration occurs. The coarser pads are great for dirt separation while the gray pads remove the ammonia from your water thru Bio filtration. The best filtration occurs with a dense colony of bio microbes which only occurs at temperatures above 50F degrees. The use of Pond Magic enzyme powder, especially in spring, can build a colony rapidly in your pond filter system. Without this early boost the colony could take 2-3 months to occur naturally..