Welcome to AA!
The only thing you can do is to remove the entire pane & silicone in a new piece. That is going to cost you a pretty penny.
You can't really silicone or otherwise fix a crack in glass & expect it to hold the tremendous pressure in the tank .... Water is over 8lb per gal
..... that is a lot of force in a large tank like yours. Even a small crack will grow & eventually leak. You can basically forget about fixing a large crack.
The only cheap way to fix the tank is to do what I did. I took the tank apart, cut off the area cracked. Cut the rest of the panes to fit & rebuild a smaller tank. <Which I am using as my new sump.. . I got a 40 gal
or so tank out of a cracked 60 gal
.> Now, this is labor intensive, and I do have a diamond coated blade (bought for cutting concrete pavers before) & a wet saw setup. I don't know how much it will cost for a glass shop to cut the glass for you .... At any rate, the dis-assembly, cutting & reassembly took several weeks (prob 20 hrs of actual work) ... but I figure I'd try something new & have some fun with an old tank that I was going to throw away.