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You gotta love those brand-new preowned tanks. Two of my current tanks are of that variety (40, 23), and my former 75 was also a new used tank. My 75 had spent its time prior to me as a home for a Python. The Python's meals had apparently chewed at the silicone in their desperate attempts to escape becoming lunch.
I had to replace all of it by removing it with a razor blade, scrubbing the areas to be sealed with acetone (you could also use rubbing alcohol--not quite as effective), and then using permatex 66br clear silicone (available at any auto parts store, less expensive and just as safe and effective as aquarium silicone). In actuality, it's the same exact product, but the label which says "for aquarium use" adds about 25% to the price.
My 40g was a different story. It would not leak until it was completely full. This, of course, was not at all frustrating. However, because it was a very small leak in one small area of seam, I was able to take a shortcut and just silicone over the existing silicone. Again,you need to clean the glass that will receive the bead with acetone before applying the silicone. When you apply the bead, make sure that it is wider than the existing bead. Also, make sure you resilicone every seam, not just the area with the leak. Sure, you'll end up with large beads of silicone, but you'll avoid the pain of removing the old silicone (and it is a major pain, believe me).
A lot of people will tell you that you have to take out the old silicone no matter what, but if you can locate the leak and it is small, you can avoid this. My 40g was done in this manner, and has been functioning for months without any trouble whatsoever.
If you do decide to remove the old silicone, be very careful to leave the area where the glass overlaps untouched by the razor blade. Your only concern is sealing the seam, not the joint itself.
Although acetone and rubbing both evaporate without leaving residue, it's still a good idea to rinse thouroughly several times before you apply the silicone. Make sure the surfaces are completely dry before applying. And yes, you need to let the silicone dry for 48 hours before filling with water.
To apply the silicone, cut an appropriately sized hole in the tip of the tube and apply an even bead to one seam at a time. Try to make the bead as even as possible. Smooth it out with a wet finger (use a bowl of warm water, and keep that finger wet). Again, even, steady pressure is the key. You can remove any excess silicone with the razor blade after it has cured. Do NOT attempt to remove excess until after the silicone has cured, or you could end up removing the entire bead, and you'd have to start over.
It might sound like a daunting task, but it's really not that bad if you just pay attention to the details of what you're doing.
Good Luck!
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