I have recently bought a Perfecto Glass Cover which is supposed to fit a 30x12 (29g) tank. I am having trouble figuring out where I should fit it. If I try to fit it on the inside rim, the cover is by 0.5 cm too long. If I try to set it atop the aquarium, it's exactly 0.5 cm too short. Am I missing something or I just happen to have the bad luck to pick the wrong cover? Is the glass cover supposed to be set on the inside rim of the tank or above the tank? I need to figure out how to set it securely since I have a double strip light to place on top of it and I don't want the light to fall into the water and electrocute my fish. Any suggestions are appreciated. Thanks!