Corplast (corrugated plastic) would work fine. I have used it to cover overflows or as an inside background for years in saltwater and not had a problem over time. It does not break down. I also use it quite often for a background. I silicone it to the back glass (outside) to prevent salt creep or water stains from getting on the back glass.

LOL, that might be a problem then.

ok, java moss then lol

Go
 

i siliconed a mirror on the back of a 54 G freshwater tank, It turned out nice and made the tank look larger
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