If you have a leak on the bottom seam, you should use a razor and slice all silicone off. I usually just slice the whole aquarium down and completly recaulk. Leave the glass glued together of course. You never really know where it orignates, like a roof leak, but worse. you may be able to find exact place it is leaking. then caulk again, making sure silicone goes into everywhere. If you are not good at it tape prior. I never use tape, but I have been doing it since silicone sealant came out. Before that with the tar stuff, and fiberglass and wood tanks. Someone posted on the Tampa site a few weeks ago, a big tank build with black silicone, and it was a real mess, very wide silicone, in black too, looked real bad. You dont want that look. if you need help, let me know, I have done this many times. Are you sure it is not a bulkhead fitting or something else? Silicone usually doesnt start leaking, unless the tank stand is wracked, or something actaully eats into it.