oh dear!!! Is it cracked? or is it just leaking around the seals?
If its just the seals you are going ot have to strip it and reseal with aquarium silcon sealant (or whatever its called). I've never done this... ive only sealed up leaks. YOu could just seal up the parts that are leaking but its better to do the entire thing .. that way your sure its not going to leak again. Also it takes a day or so for that stuff to cure so you want to put your heater and an airstone in whatever container you are currently keeping the fish in.