I get calls from folks with pond problems. Here's one I got recently. A woman has a concrete pond with a huge crack in it. She hired a local pond professional to repair it and he guaranteed the repair. Neither she nor I know what he did, but he did not fix it. He has been back to try again a few times, but has failed each time. He now wants more money to continue to try. I told her that in my years of experience, you never, ever guarantee the repair of concrete. Most of the time concrete cannot be repaired. Just look at our expressways to see how brittle concrete is.
Our ground is always moving, water comes up and recedes, concrete freezes and thaws. Sadly it is not plastic, it is pretty solid and eventually cracks. The ONLY way I know to repair a crack in a concrete pond and this is definitely not guaranteed is to scrape out the crack, get it clean and dry and then use Plumbers' Epoxy and mash it in the crack. Make absolutely sure you are wearing rubber gloves while using Plumbers' Epoxy or you will be wearing the epoxy for weeks.
Remember this is not guaranteed, but it's the best I know.
And if somebody guarantees a concrete repair, do not believe them. They have not had enough pond experience.