To heat your pond for Barramundi during cooler months you can add a heater: Heaters
Using poly-carbonate sheets over the pond, while allowing air flow, will help trap some warmth in too.
Otherwise the ultimate way to keep and grow Barramundi all year is investing in a greenhouse – http://sproutwellgreenhouses.com.au/
If you are interested in purchasing a greenhouse just contact us and we will organise it for you.
The key indicator for Barramundi under stress is a pronounced white stripe on the head and lack of appetite – test your water parameters, including temperature, ensure the pump is clear, and/or air pump is working. Note the white stripe is temporary and also normal when being transported into a new environment.
If there are abnormal levels of ammonia, nitrites etc, do a 1/3 water change, (make sure you condition the water if from tap), and make sure you have lots of splashing water or use an air pump. You will need to keep doing water changes until the levels are normal and temperature is correct. If they are still lethargic and not eating after 24-48 hours, do a salt treatment.
Any signs of fungus, white films on skin or fins, or gasping, do a salt treatment – See Library – https://www.woodvalefishandlilyfarm.com.au/medicating-aquaponics-fish/