I have successfully patched terrariums and made them waterproof again. both side panels and even bottom. The side panels are pretty easy. I just scraped off enough silicon to get the fit as tight as possible and then I put jesustape as we call it (roadietape or duct tape) on the outside to keep the cracks in place, then I just siliconed the new glass to the side both framing the pane and criss crossing it. I used some duct tape and a rather large stone to keep the pane in place while drying. Later I siliconed it from the inside to make sure it was tight and presto! As good as new.

On the other tank where the bottom went crack in the night, I ordered a new pane cut to fit inside and did the same procedure. working well still ( keeping in mind that I have about 8 cm of water covering the bottom of a 60d 95w 70h tank...)