You shouldnt really need to replace your sand. I have a "python" water changer that I use to siphon the crap off the surface of the sand. Then I occasionly lightly stir up the sand to release any gasses that build up under the sand over time.
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Currently in the US Navy so I have sold off my collection. But over the years I have had dozens of turtles, fish, geckos, and other lizards. Cant wait to get back in the hobby once im out though!