The only option is the take the tank completely apart. I have used the blades from disposable razors to cut through the seams. They are very thin and flexible, some have had luck using very thin wire and pulling it back and forth in a sawing motion. Before you reassemble the panels you must clean off all of the old silicone and its residue. Scrape with a razor blade then scrub with steel wool and acetone. Putting it back together is not all that difficult but it does require patience.
1. Lay out the panels and tape all of the edges that be on the seams. Hold the tape back to create about a 1/16" space and remember to make an allowance for the panels that have an opposing panel abutting them. The reason for this is to make a nice straight internal seam when you spread the silicone with your finger instead of an uncontrolled wide ugly seam.
2. Spread a nice even bead of glue on seams. It only need to be on one side of each seam. Do one panel at a time and have someone help you to hold them in place while to apply the silicone to another panel. It is critical that you work at a steady but fast pace. DO NOT let the silicone skim over or it will not bond correctly. You have about 5 minutes per panel. Once you have the panels set in place and lightly pressed you have plenty of time for the next step because air cannot get to the silicone.
3. Once the panels are in place GENTLY push them together and check to make sure there are no air bubbles. DO NOT push them so tight that you squeeze out all of the silicone or you will need to start all over. You only need about 1mm of silicone in the seam. You will need some means to temporarily hold the panels in place. Clamps work good, either bar or strap. You also simply run two or three pieces of tape all around the perimeter of the tank. The critical thing is to not clamp the tank so tight that you squeeze out too much of the silicone.
4. Smoother out the silicon oozing out of the inside seams (hopefully you did the tape off technique). After you have removed the excess silicone remove the tape before it dries and you will have a nice clean internal seam, just like the pros.
You tank probably has a max of 1/2" glass. My guess is you can get a piece that will work for about $50-$75. Measure the old one carefully and make sure they polish the edges. I have only used RTV silicone, you can get either clear or black. I would estimate no more than two 10 oz tubes. You will likely need to order this online, I have never seen it the big box stores. So for under $100 you will have a nearly new tank.