Man I hope you have some place to keep the critters while you repair it. You need to remove the broken pane and all the silicone glue that surrounds it. Make sure you do this second task well as new silicone does not adhere to the old dry stuff. I would then get the same thickness of glass cut to your tanks dimensions. I usually then use a thin coat of water proof epoxy to glue it in place then, when that dries, apply a silicone bead around the new pane. You can use only silicone if you wish but the two glue system is easier to apply in my opinion.
Good luck Burtone.