The most important thing is to get ALL the old silicone off the inside of the tank, so there is only a very thin layer left between the glass panes (obviously you can't remove that). Use a sharp razor blade to do this, then remove any residual silicone scrapings, and clean all the surfaces thoroughly with isopropyl alcohol. Let it dry thoroughly before applying new silicone. Silicone sticks to glass; it does not stick to already-cured silicone
You'll need at least two 10 fl. oz. tubes of high quality aquarium sealant for a tank this size- the Aqueon product is available through Amazon (
http://www.amazon.com/dp/B0002ASD34/?tag=wafishbox-20). You will also need to buy a calking gun. With a little practice, you will be able to lay a nice uniform bead of sealant along each seam. Do the whole tank at once, before any of the seams have cured. Let it dry thoroughly before doing a leak test (outdoors!) Good luck.
