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My uncle has a 9000 gallon pond. Every year we put 5 or 6 plecos in the pond. At the end of the year though we bring the scale less koi indoors so we lower the pond and can catch the plecos. They wont survive the winter here though. We can do this also though because there is a hot tub heater hooked up incase it is needed. We also put 2 oscars, 2 pacus, and 10 tinfoil barbs in there each year as well. The heater is so the water won't drop below 60 degrees so that the tropical fish won't die. Then we bring them inside in the winter and put them in a pond that he built in the basement.
So the answer is you can put plecos in your pond. Just make sure that you are able to catch them before the winter and that you keep an eye on the temp of the pond to make sure that it doesn't get to cold. Just so you know we only put common plecos in the pond cause if for some reason they do die there cheep. Hope this helps.