Rhino 3" Retro Bottom DrainRetrofit Bottom Drains offer a quality innovation for a stable and rugged over the liner bottom drain for those who do not want to install a drain through a liner, or for pre made ponds that do not have a bottom drain. This drain is especially useful for pond and aquatic environments that have preexisting liner, concrete, or any other situation that would not allow underground plumbing to be installed. Keeping solid waste and debris out of your pond