picture would prob help. But if its the one I'm thinking of, you put the piece of metal which looks a bit like a hook over the edge of one of the pieces of glass on the front of your tank, and tighten the screw on the back. Then close the glass doors and there will be the "teeth" of the lock sticking out from in between the inside and outside pieces of glass, parallel to both. Then the lock part goes on this other piece, turn the key, and hey presto, it should work.
Difficult to explain, esp when I can't see what lock you are actually using! Did the lock not come with instructions?
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