I have a prefab pond, does anybody know how to fix a leak

Yes, but it is some work. Plumbers goo, it was also known as shoe goo. It's a
thick goo in a tube and it works well. You need to fix the crack cause, such as
a rock pressure point under the preform. Scrub clean the crack and apply the
goo. Let sit for a day and refill. I use goo for rubber liners as a cement for
a patch, it seems to last forever.