You will need to remove all the silicone on the inside with a razor, then clean it real well. You should never put new silicone over old, it won't bond together that good and you will have a chance of a leaky aquarium again. You don't need to dig the silicone out of the seams just cut and scrape it off the glass surface. Also make sure you use silicone that doesn't have mildew resistant additive, just plain silicone. When I resealed a tank I used all-glass aquarium silicone just to be sure. After I was done scraping I cleaned the seams with acetone to remove the film and then cleaned it with vinegar and water real well. It will take some time and effort but I'm sure it will be worth it.