New silicone will not bond well with old silicone, so you will need to monitor any patch job carefully. Depending on the size of the tank, you may have to strip the inside of all silicone and then reseal the entire inside.
You can purchase replacement molding from either All Glass Aquariums or Perfecto. You may also be able to purchase one from your LFS
, but they will likely have to special order it.
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