I get questions all the time about leaky ponds. Since there are so many kinds of ponds, so many kinds of leaks and so many ways to repair them, I want to spend a few days talking about leaks.
Let's start with concrete ponds. I do not recommend building ponds with concrete because it is not watertight and is brittle, but many people use concrete, so I get to fix their leaks.
Concrete ponds crack. It's inevitable. If you have a concrete pond, it will crack. Plan on it. I don't care if you use concrete 6 inches thick and 6000 psi. It's gonna crack. So, now what?
If the crack is hairline, you perhaps can fix it. Maybe. For a short while anyway. Get some plumbers' epoxy. Not plumbers' putty. Epoxy. Make the crack bigger. Dig it out so there is a bit of a V shape, like the dentist does when he fills a cavity in your tooth. Now make sure the crack is dry and clean. If you have to, use a hair dryer to dry it completely.
Now mix the epoxy. Put gloves on first because the epoxy sticks on your hands just like it sticks on the concrete. Epoxy comes in a tube and looks like a big stick of chalk. So unwrap as much as you need and start pinching and rolling it - gloves on, remember. In a couple of minutes, the epoxy will get warm. A minute or so after that, it's ready to use. Smush the epoxy in the crack. Notice the technical term, smush. It's important. Make sure the epoxy is pushed into the crack and smoothed out both in the crack and along the edges. Wait for it to dry. The directions on the side of the package will tell you how long that will take.
When the epoxy is dry, refill the pond with water. The epoxy will not hurt your fish or plants.
I have had some repairs last for several years, some for only several months. I never guarantee a concrete repair. Often I will do it free if the pond belongs to a steady customer. If the customer pays for the service, he expects some sort of warranty and this repair is not good enough for that.
If you can see through the crack in your concrete, you cannot repair it. I don't care who says you can, it is not possible. Find a waterproofing company and have them put a new coating on your pond. Or just drop a butyl rubber liner inside the concrete pond and start over.