stick another glass or plexiglas inside and seal it on top of the broken one, maybe do the whole bottom or just that corner...dunno for sure, having to keep in mind about weight and pressure and so on. But I would do a complete bottom on top of the cracked glass
but in my logic world it will work just ok like that.
I did the same thing on my used 30g i bought yestrday, i tried to replace the glass, but I asked my Father to cut away the silicone (because i sliced open my finger) and he ended up breaking the whole **** tank. Oh Well it was only $10, and the store where i got them has 8 more.
once a crack appears, the structural integrity of the bottom has failed. The crack will keep getting larger and larger.