Here's How:
Locate the exact source of the leak and mark it with a felt tip marker.
Reduce the water level in the tank until the leak stops.
Thoroughly dry and clean the area of and around the leak with a paper towel, or soft cloth.
With the razor blade, scrape the existing silicone from the leaking area, and an additional one inch area around the source of the leak.
Clean the scraped area with the acetone applied to a paper towel, being careful not to over staturate it or allow any acetone to run or drip down into the aquarium, and let dry for a few minutes.
Apply the silicone sealant to the cleaned area, working it into the glass joint area.
Allow the silicone to cure for at least 12 hours, 24 is better.
Refill the tank with water to the desired level.
Check for leaks.

Tips:

Not all sealants are suitable for aquarium use. Use only non-toxic 100% silicone sealants, such All-Glass® brand.
Pinpoint the exact location of the leak. Water will be at the bottom of the tank (gravity rules), but the source may be somewhere else up higher or sideways along the joint or seam.
If patching the leak from the outside of the tank does not stop the leak, try patching it from the inside.
If patching the leak from the inside does not work, follow the diretions for making a major leak repair.