[Morph]

is it being used as an aquarium?

If you are filling it with water then u will need to replace the glass completely with toughened glass suitable for the wieght of the water.

if you are using it as a terrestrial set up then just stick a load of silicone over the cracks.

You can buy animal safe aquarium silicone from most pet stores or fish stores quite cheaply so i'd be on the safe side and use that instead of diy sealant.