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One of the best products we found for bottom drains came from Kockney Koi in England.  The actual product is produced in Japan by Yamitsu .  As you will see from the photos below the flange area is quite large so there is contact with the liner.  The various sizes all work the same way.  You can get the device with a dome on it which keeps fish from being sucked into the opening.  The larger four inch bottom drain which is what is pictured here would be used as a drain in the lowest portion of the pond.  In a smaller size such as two inch, the fitting can be used as a side drain if you want to pull water from a certain part of the pond at a certain depth. The smaller ones are called Tommy Drains and come in sizes 2”, 3”, and 4”

I have the 4 inch and 2 inch models.  Since these are metric sizes, each fitting comes with a conversion piece so it is interchangeable with US pipe.