The Helix Skimmer uses the same technology as a pool skimmer, but yet in a pond. The skimmer can be used with a submersible or external pump. Fish can swim in and out of skimmer without being trapped. It is constructed of a heavy duty HDPE mold plastic. It comes with stainless steel screws to attach liner to the outside rim of the 12" faceplate. A 2" bulkhead is included to be put in the bottom flat back of the skimmer. There is a pre-drilled hole in the back for a external or submersible pump.(Reason for the pre-drilled hole is so the costumer does not need to buy a $60 hole saw drill bit that they will only use once.) Has an 8" x 8" x 8" basket that debris is compacted into due to the design of the Helix Skimmer. ..