Yeah I'm having the worst luck - my 3ft tank has a massive crack in the bottom and I need to replace it - now this leak in my smaller tank :-/ hubby is going to try to sort it out when he gets home. Crappy day!!!
I had to reseal my first tank. I was brand new to fish keeping and did lots of research before hand. I just took a razor blade and scrapped every bit of the silicone off. I spent two days going over it. New silicone will not stick to old silicone. Then I cleaned the corners with acetate, finger nail polish remover. I used 100% clear silicone with no additives for mold preventer. The one thing I wish I had done was use masking tape and tape about a half inch from each corner. That way you get a nice clean silicone line. I just squirted it into the corner seam and wet a finger and run it along the line to work it in. It left a bit of a thicker and messy corner but it has been running for almost 2 years and no leaks.