Prior to the 1970s almost all pond drainage and overflow systems were made out of galvanized pipe. A pipe was embedded during construction at the bottom of the pond with a standpipe protruding upward to the desired level of the water. These systems preformed adequately but the had some serious short comings.
- They rust out. Galvanized piping, even when efforts were made to minimize it, have a very limited life span. Typically this might be 15 to 20 years. Replacing it without digging up the pond is almost impossible.
- They drain water from the top of the pond. The top couple of feet contain most of the nutrients for you fish and since the pond drains from the top, much of the nutrients are swept away with the drain water.
A new system provides answers to these problems and many pond builders have been installing syphon systems in both new ponds and as replacements for old galvanized systems.
The advantages are that when made of Schedule 40 PVC ( the same stuff your plumber uses ) the piping is virtually weatherproof for an untold number of years. It drains from the bottom so little or no nutrients for your fish are flushed away. Since the structures are glued together it is fairly easy to build. To replace an existing galvanized system you must seal the old piping completely. This can be done by stuffing sacks or other bulky items into the old pipes and then using submersible concrete such as Sakcrete into the pipes until you get a good seal.
The only real disadvantage to the siphon system that I can see, is when draining the pond it is almost impossible to get to the very bottom of the pond. Since you are left above the bottom you cannot completely drain the pond. If you are replacing fish you may have to use a fish kill to totally start over with your fish population.
A complete description of a Pond Siphon System is available.