drain the water...let the tank dry.
get some aquarium silicone...and cover the crack with it...
let it sit for a week and fill it back up
or even harder go get a new pane of glass, peel the silicone out of all the corners that connect it, acetone all edges adjacent to the glass that is cracked ensuring there is no silicone left....then slowly and steadily silicone the new glass pane to all edges of the aquarium let it sit for 1 week...water test it