Question
Hiya,please can you help us? We have rebuilt our pond, we used a pondliner,natural slate which was bedded on with a weak solution of sand and cement.The pond takes approx 1,000 gallons of water, this was made up of approx 1/3rd of original pond water and the rest was tap water. The tap water was left to settle for a few days before we added the old water and fish. We lost the fish overnight.So unhappy! Having visited a specialist on ponds, we found the ph was over 10. We have drained the pond 3 times and refilled with tap water. The tap water is reading 7.5 ph but once in the pond it increases to 10 plus again.We have also used Oase ph minus solution, which has not helped. Someone in a pet shop suggested using crushed oyster shells in the filter, this actually made it much worse. The plant life seems to have survived this catastrophe, but we really wanted fish.Hopefully you may be able to help us.

Answer
The reason for your high pH is high alkalinity (KH).  Test that and see where it is.  It should be between 3-5 ppm.  Test the tap water's alkalinity (KH) while your at it, just to rule it out as the problem.  It is most likely one of the building materials you used leeching into the water.  Make sure every surface is properly sealed.

Crushed oyster shells, crushed coral, aragonite, etc. will all raise the alkalinity quickly and efficiently.

You need to first discover the source of the problem, eliminate it, refill with water, dechlorinate, add fish, and see the pond through its nitrogen cycle.

You should be good after that.