Ira is right get a secondhand replacement or even new cheap.
If you want to fix go to a glazier, some will cut a replacement and glue it in. won't cost to much.
Or go to a house demolition place, buy a piece of glass and cut it yourself. have to buy silicon and a glass cutter, but is fun if DIY interests you.
Do Not try and repair the cracked glass, also there must be a reason for a tank to crack, the glass may be stressesd somehow, check the new piece of glass fits well the tank is level and on a flat surface.