When I first got my used 55g it leaked as soon as it was half full. My
husband took a razor and removed all the old silicone to the glass seam.
Since it is nearly imposable to find the exact spot it would be a good
idea to re-do the entire tank. Use only aquarium safe products and
follow the directions. One thing that I did was use a latex glove to
smooth out the wet silicone to get the best seal.

It is not hard to do and in a few days your tank will be as good as
new.

Vicki