How to fix a leaking fish pond?
I may give you some news you do not want to hear, but here goes. The process you used probably did not work and you probably have many cracks in the cement. Sealing the cement will not repair the cracks. When building a pond from cement you need steel reinforcement in the cement. Then to make the pond water tight you need to seal the pond with plaster. There is really no good way of repairing your pond at this point. Your options are to leave the cement in place and cover it with an EPDM pond liner. This will work as long as you don’t have any sharp pieces of cement pocking holes into the liner. The other option is just to start from scratch and remove the cement and place the pond liner in the hole. Sorry to give you the bad news, but using the EPDM liner is really the best way to go.