A day at home today gave me the opportunity to check the pond and the ranchu over in the daylight. A little wall algae waste had accumalated but this was syphoned out easily.
The activity inspired the ranchu into investigating what was occurring. Whilst not ideal, it did give me the opportunity to get a closer look at the condition of each of them. They all look well, which is reassuring, given that they have now spent a couple of full weeks below 10 degrees C, with the ranchu barely moving at all, which is still a little weird as at any other time it would signal an issue.
The pond has been sitting at around 7c, with the current outside conditions and the lid keeping temperatures just above what the thermostat is set to. We have colder conditions forecast and i may lower the thermostat a little further.