I definitely would not just silicone over the old silicone. New silicone doesn't stick to old silicone. Especially if you see an active leak... means the internal seal between the glass panes have been compromised so you would have to remove the pane of glass entirely, scrape all the old silicone off, and then reseal the entire pane of glass. To do that though, you would have to remove the trim which is just as labor intensive of a project... at least it was for me personally. Then you would need to replace the trim or get a custom one made. I ended up getting a custom stainless steel trim made for my tank with two cross braces.
IMO, for 240 gals, unless you trust your ability to reseal glass or someone else's ability to do that, I would look for a new tank and pass that project along to someone else that has resealed tanks before or doesn't mind the risk of 240 gals spilling out in their living room.
I say that as a person that resealed the front panel of his 215-gallon tank. It was very labor intensive, very nerve-racking the entire time, and then when I finished it, I had to get a custom stainless steel trim installed on top as I started to see the panel lean out from my seal job from the pressure of the water. The new trim did it and kept it nice and tight but I still wonder sometimes how long it will hold up in the long run. Do I have 3 years? 5 years? 10 years? 1 more year? It's been sitting pretty for 2 years now.