Tony CM is correct again, and the water level will be 5/8" lower than the top once this gap is plugged.
Suppose that the aquariums 24" tall (and it is). If the front panel was 24" tall both the top brace and bottom panel would be glued to the inside this panel. If the front is cut 23 1/2" high (1/2 inch glass) then the front panel is glued to bottom panel in the same manner but the brace is glued to the top of the front panel, which is how my tank is. This is ok as long as there is not a small gap in the brace.
My overflows are out of 1/2 inch glass and I would have to trim an even amount off both of them
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I think I am going to have to silicon the gap, and then let it dry. It is only a problem when I place an object in the tank, and it causes ripples on the top of the water.