When repealing my 200, I took all of the old silicone out,
Even the stuff at the top, because new silicone won't adhere to old.
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Anyway, I agree with petluver. I have never re sealed a tank either. But from what I have read you are supposed to remove all of the material and replace it. New doesnt stick to old. So if your water was up to the top for any reason, it could leak easily.