is it leaking at the seal or is the glass cracked???
if its the seal your best to
1) empty and dry the tank
2) turn the tank upside down and using a long craft knife remove the bottom panel from the tank. leaving the 4 sides standing (by long craft knife i mean longer than a stanley knife)
3) clean all the old silicone off the base and off the edges of the 4 side pieces of the tank
4) run a good thick bead of silicone around the top edge of the standing part and then ease the base back onto it pressing it down firmly...
5) turn the tank over trying not to move the silicone and wipe of any excess from inside the tank...
leave it for the silicone to cure (usualy 24-48 hrs,) then test fill it if its formed a good seal then your fixed.. if not re do it