So, what if your pond didn't get dug with a bottom drain or you don't want a permanent drain in the lowest part of the pond? These drains meet your needs.
Retro-Fit or "Through the Wall" bottom drains are simply bottom drains that you can plumb from a bulkhead in the side-wall of the pond. Most folks can dig down a foot or two by their pond, to get under the water line and install a bulkhead in the side of the pond. This then plumbs to hard pipe and down to the bottom of the pond where a bottom drain should go.
You should plumb these onto the appropriate size pipe with a Fermco rubber fitting. And you're well advised to plumb these onto hard pipe because they are slightly buoyant because they are made of polyethylene. If you plumb them onto soft or flex pipe, they will slowly float up to the pond surface. So, plumb these onto hard pipe and they'll stay where you put them.
By installing these on hard pipe through the pond wall, there are several advantages.
First, there's no way for the pond to drain completely. With a real bottom drain in the bottom of the pond, at the deepest part, if the bottom drain or the plumbing after it were to fail, the pond could be drained 100%. With these, when the water line drops below the bulkhead in the SIDE of the pond, the airlock is lost and the pond does not drain out.
Plus, since all the plumbing for these Retro fit drains is above the drain, it won't fill with rocks or sand over the years and end up clogged. Simply clean it out.
These are durably constructed and are very effective price wise.