The first thing you need to do to tackle leeks in a pond is to establish where it is,you can then begin your structural repair.
Firstly turn off any pumps to stop the water circulating, if the water level drops to a certain level then the leak is in the pond and not the pump or joints.
Eventually the water level will hopefully stop, and you will be able to establish at what level the leak is, if the water continues to empty then its either on the bottom or (start praying) in the drainage system.
Check any pipe joints as these are prone to leaking.
When building a pond with a drainage system, make sure you use good products, to hopefully stop this problem happening.
If you install a bio filter make sure its accessible and situated over the pond, if it does leak out of the lid due to blockage at least the water will still flow into the pond, its better to have dirty water rather than no water.
As you will be aware if there are any holes or gaps in any waterfall system, it will find it, and cause problems, thatís why itís always better to get a preformed waterfall rather than trying to create one yourself.
In my experience pond liners or preformed ponds are much better than concrete, although you can reseal concrete mainly using fibreglass resin or propriety bond sealant the failure rate is very high, and stone walls can be plugged with specially made foam spray, at least with liners or preformed they can be patched or even replaced.
Remember if you are building your own pond, plan it out well and use quality products and fittings,if not you may be carrying out structural repairs before you know it.good luck.
Return from structural to maintaining ponds