Just a small update on the current condition of the ranchu and the pond. The pond temperature has continued to drop gradually and is now sitting at around 11 degrees C, so not much farther to go. Meanwhile, the ranchu have been looking well, and, as active as i would expect, given the temperature.
Another Christmas present arrived recently, which will be very important when i begin to warm the ranchu back up, after the winter - An external digital thermostat. Unlike heaters which each have individual thermostats and generally operate above 16 degrees C, the external thermostat operates from 0-50 degrees and will control all heaters. This will be hugely beneficial at the lower temperatures, when still wanting to increase or decrease the temperature of the pond gradually.