Absolutely test for leaks first. If it doesn't leak, you have saved time and $$. I stored a 29 gallon tank for over 17 years, moved it over 7 times, carried it 1200 miles in a U-haul on two different occasions, and it's in fine shape (currently stocked with guppies). Even has some chips on the edges, but never leaked a bit.
If you have to reseal, are you contemplating removing the existing silicon? I'd recommend just to seal over the existing. You can seal over existing silicone, it just depends on how bad the original silicone looks. You can also spot-seal only the leaky seam/area. The best way I have found to remove existing silicone is a single-edge razor blade. Be careful, it's very, very, easy to slip and cut your fingers badly. A utility knife can work well, and more safely, for thicker areas of silicone, but be careful not to scratch the glass. You'll have a much harder time getting the thinner layers of silicone off (that's when I'd use the razor blade). There will be some silicone "gluing" the glass panels together, so you can do all sides at once. I wouldn't recommend moving the tank around if you remove all the existing silicone, until you replace it and the new stuff cures. Don't remove the bottom silicone, just thickly layer new silicone over it - no one will see it beneath the substrate anyway, so don't waste time or caulk! Also, unless it's come off, I'd leave any plastic trim alone, I would not remove it and glue it back on just leave as is. Particularly don't mess with any center brace!
If you reseal or seal over, make absolutely certain you use 100% silicone. You can purchase a large tube and a caulk gun at hardware stores. You'll need to avoid any type of bathroom/window/tub caulk. Those have mildew inhibiting chemicals that are toxic to fish. Techniques to apply vary. For a really neat line, I put tape on each side of the area I want to caulk, then caulk messily down the middle. Since I can never seem to get a really smooth initial application line, I don't waste time trying. After applying the caulk line, I wet my finger, then smooth out the line of caulk, making sure it's really thin along my tape edge (for aesthetics). While the caulk is still "wet", I pull off the tape - voila, a straight, smooth line. You'll need a plastic cup of water, and some wet rags to wipe off your fingers and fix the occasional "oops". Basically, you are just filling in the "V" formed by each glass edge. Wait at least 24 hours before moving after application. Just to be safe, I'd wait two days or more to allow the silicone to fully cure.
Hope this is helpful!
Last edited by DKRST; 03-17-2011 at 02:43 PM..