Today the Japanese ranchu pond has dropped to a temperature around 8.5 degrees C. A couple of weeks of colder weather is forecast from next weekend and so I think the pond will reach its target temperature of 6 degrees easily in the next few days.
I decided that i would syphon out the little bit of waste that was present in the pond and top up with water that has been matured. All in all, i guess around 30% was replaced, with just under 100 litres of fresh water going in. I intend to leave the pond in its current condition for at least four weeks with no water changes and no feeding.
Whilst syphoning, the ranchu were inspired to move about a little and investigate what was going on. All appear to be healthy and fingers crossed, they will be OK over the next few weeks.