acrylic does let more light through but does warp with heat like Nate said.My 30 and 20 gallon tank dont have a cover so I made them with acrylic.
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This is how I cut mine and glued the over lapping acrylic with aquarium grade silicon. There are three pieces. The two larger ones on the side, and a long horizontal one. They are overlapping about an inch. Notice the opening in the top right corner, that is where the cord for my heater would come out of. The opening in top was for my filters and the one up front was to easily feed them without having to take the lid off. It is strong and wont warp as easily cause of the over lapping. It aint pretty but it kinda sits flush with the top off the tank if cut properly.