We have a pond in our garden built by the previous owners. It has a strong plastic liner built within a brick frame. After many years, the pond is leaking considerably but we can't find the leak. We've turned off the pumps, cleaned all the filters and the water levels have continued to reduce. We've allowed the pond to empty (moving the fish) but still can't find the escape route. It needs gallons of water daily to top up the levels. Is there anything we can try ie. bentonite (will this work on this style of pond) or relining or something else? Thanks.