In many parts of the country, the weather fluctuates wildly, and changes of more than 5 degrees higher or 2 degrees lower in a pond during a 24 hour cycle can be very stressful on the Koi! Here are some suggestions to help Koi though periods with fluctuating pond temps, by Fish Vet Richmond Loh of Australia.
Changing weather can mean sick fish. Things you can do to prevent fish diseases?
With pond temperature variations, you have to adjust how much you feed your pond fish based on the weather forecast. Reduce the amount you feed if the weather is going to be cooler.
Your biofilter will also ‘feel’ the cold and reduce it’s activity and this is another reason for reducing feed.
You should also be checking the pH and KH of the water to ensure it remains stable.
Make sure you remove all the accumulated, dead and decaying matter from the bottom of your pond and from the filter chambers.
The addition of vitamin C (available from https://thefishvet.com.au) at a rate of 1-10mg/L is also helpful to boost your fish’s immune system and will help with wound repair if they sustained minor injuries.