The radiator in the conservatory has been turned off and the door to the main house is kept closed, making the conservatory and the ranchu pond remain fairly cold, around the clock. The new thermostat controlling the heaters is proving to be invaluable. To ensure a gradual decline, i've chosen to reduce the temperature of the pond by 0.4 degrees C every day. The pond is currently sitting just below 10 degrees C and the ranchu have now slowed down considerably.
I may carry out one more water change on the pond, which the ranchu will then be kept in for the remaining wintering period. I would prefer to not stress the fish given their current state, but i have noticed a small amount of waste in the in the pond. Therefor, Ammonia and nitrite levels must be considered, with the ranchu being kept in the same water for a number of weeks.