Will the concrete need to be sealed for a new fish pond and if so what would you seal it with that won't kill the fish?

To avoid leaks there should have been a waterproofing chemical added to the concrete during mixing. If this was not done, there are products that are available at Hardware Stores and Paint Stores that will waterproof the pond. I can not name the products but an assistant in the store/shop should be able to advise you.