Concrete ponds are notorious for springing leaks. The main reason is either ageing, or poor construction - materials, methods etc - in the initial stages. Concrete Ponds in clay soils are particularly at risk, because of the inherent nature of clay soil.
Clay soils shrink when dry, and expand when wet. This constant shrinking and heaving of the soil surrounding the pool cause the pond either to break its back, or sometimes just disturb a week point, leading to the inevitable leak.
Spotting where the leak is on a concrete lined pond is normally quite simple. As with a plastic liner, the water will eventually subside to the level of where the leak is. (Sometimes, if there is a high water table in the surrounding soil, then the water will only subside to that level and maybe not down to the actual pond leak. This is rare these days with reduced water table level.)
Once the water has settled in at the level of the leak, then take out a few more inches of water - especially on a hot drying day. The leak - normally a crack - can usually be seen by the wet tell tale marks on an otherwise dry concrete surface. Once the leak has been isolated, then proceed as follows.
Firstly, it will probably be necessary for you to drain the entire pond, for there will be quite a bit of debris falling into the pond, together with wet cement. neither of these will be good for fish!
By David Hughes - firstname.lastname@example.org