Sounds like the tank is old if the plastic is cracked , usually there is a production
Sticker on the bottom .
If the tank had no leaks I would cut all silicone out with out cutting between the pains of glass and seal it up ,home hardware sells 100% silicone that states aquarium safe.
If you want David at Aqua Gaint can cut small prices of glass on the spot and ,you could urobrace the top.
5mm glass is thin for holding 350 lbs of water .
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