If it is a very slow leak, then the best approach is to simply strip out and replace all the exposed silicone inside the tank. This takes a bit of time as you have to scrape it off with a carpet knife blade(s), clean the glass really well and then re-silicone with aquarium friendly silicone. That's the most difficult part - getting a clean neat finish! You can go on-line to get caulking tips as ideas about how to proceed as the process is much the same.
The alternative is to consider a repair - Fish Addicts in Langley repairs tanks at a reasonable price (about a dollar/gallon is his fee).