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I have a 4000 gallon concrete koi pond. In the past I have been taking my turtles in for the winter. They are now too big to fit in their current aquarium and I want to try to leave them in the pond for the winter. I live in Toronto and we heat the pond to 40 Fahrenheit for the winter to prevent the pond from freezing. I understand that the turtles need a place to hibernate and because the pond is concrete I am looking for suggestions to allow them to burrow themselves for the winter. Looking for suggestions on what to build or place in the pond to allow them a place to be comfortable for the winter.

It'll be best to contact local keepers to see what they're doing to confirm what you want to try. Usually older RES can hibernate (brumate) underwater and there does need to be some sort of substrate (sometimes mud/leaves). You will also want some sort of aeration.