Problem with a covering of plexi right on the tank is that it will a) hold the heat in and b) get splashed or covered in moisture...as such blocking the light into the tank.
You would have to be religious about cleaning the top up if you thought that heat wouldn't be an issue.
I have heard on different occasions that it is said to interfere with proper oxygen exchange as well but I'm not too convinced on this.
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Mark...
290g Peninsula Display, 425g total volume. Setup Jan 2013.
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