We used cement board for both sides of our tank wall. You have to use different screws and it is tough to cut but no worries about water damage. The front side is sealed with caulk and expanding foam and the stand and tank are slightly sloped to the back to help keep water away from the wall if something happens. For paint we used Cloverdale Eco-logic the stuff they use in hospitals. It's acrylic so no fumes but it's waterproof like enamel.
HTH
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