If it's a seem leak , you may be able to fix it with the last posts instructions , if it's cracked , the crack will most likely travel [ get longer ], under the pressure from the water inside pushing out , and could burst [ or just leak out ]causing you more problems then it's worth [ esp if you're not around when it happens].
You can remove the cracked glass and replace it , this too is not without risk however .
And easier said then done .
Sorry , not many people have good luck with leakers , I'm not going to say not to try , you could be one of the exceptions .
I would suggest leaving it full of water only , after fixing , for like a week at least in a safe spot, like outside if possible .[ somewhere water damage wouldn't be costly ].
B)
I honestly don't think a cracked tank is worth the time or expense . B)
They would make great reptile tanks though.
silicon sealer should be for aquarium use , some silicons made for home use contain mildew inhibiters that may cause harm to your tanks inhabitants .