Hello there. we have a acre pond that is 13 ft deep,after our pond was full after filling it , it started to drop at the rate yours is. We bought a pallet of bennanight of granual size pellets and spread it . After spreading it and letting it swell we drug a chain all around the pond stirring it all around. It has never leaked again. We also have our pond full of trout and it is harmless to your fish. Good luck