I don't know how glass replacement would be successful for very long. After removing the broken panel (no mean task), you have to remove absolutely every bit of old silicone, too. New silicone will not stick to old silicone.
I'm finishing up a boat project, and removing old silicone from a smooth gelcoat surface was a complete cast iron b_tch of a job. Removing silicone from the stern bracket of the engine was a complete PITA. Silicones made for below-the-waterline sealing are tenacious as anything (unless applied over old silicone...then they come off easier then you want them to). I don't see how you could get it all off of the edges of the remaining glass.
Your options: you may want to pay an LFS to do the work (if they will and will guarantee the work) or trade the tank in for a new one.
I'm not optimistic about replacing only a single pane on a tank that big. Water damage from a leak could easily exceed the savings achieved on that repair.
v/r, N-A