I think if I was doing a pond , I'd be letting it leach for a while and giving it an acid wash before I considered sealing it. This gets rid of the lime that will kill the fish otherwise.
When building cowsheds we used a product called isinglas, used in the brewing of beer for clearing the fines, and my Mother used it for preserving eggs.
What happens on concrete is that it goes on like water, and when it is dry, the concrete is covered with what is easiest to discribe as, glass.
This can be done several times with it drying out between coats.
If the milk can't eat it then it MUST be good.