In many ways, Autumn is the most challenging time of the year when it comes to temperatures. In Winter we expect the fish to be hibernating and in Spring we want to want them up while in summer warm temperatures are generally desirable. In Autumn however, we are looking to cool the fish down but as Autumn can be volatile a few cool days early on can be followed by a warm spell before a period with frosts – and we are looking to gradually reduce the temperatures.
The gradual reduction is so the Goldfish continue to feed and develop while recognising that the temperatures need to fall as a sudden drop can be harmful. If that downward slope is affected by a warm spell of weather you need to adjust your plans accordingly.
This is where an external thermostat can be very helpful. The typical aquarium heater / stat has a lower range end to its range at about 18C – 20C. Given we are looking to control temperatures down to say 4C – 5C, there are a couple of ways – using an under-powered heater and maybe restricting the time it is on – or an external thermostat. This is the method I use and it means the Goldfish can adjust over a period of several weeks to cooler water.
Exactly how low a temperature you want to achieve in the Winter may depend on your budget; the varieties you keep; your plans to breed your Goldfish next year (and when next year) and other factors; so each person needs to plan this for their own circumstances. I look to go from about 16C at the end of September to about 5C in mid December so about 1C per week, on average, maybe a little more if the water is warmer in late September, or maybe a little less if December is still mild. As I say, this is one of those areas in Goldfish breeding where the local weather has significant influence, no matter what we might want to do.