I have been using a soil thermometer with an 18 inch probe inserted though a plastic fishing float and a nylon cord. Found the soil thermometer at Gemplers (Gemplers.com). The current issue of Pond Boss magazine has an article with reference to a digital thermometer that can give readings up to a depth of 23 ft. This would be most helpful to determine the thermocline. I think I'll replace the soil thermometer with the digital temperature sensor.