Murray Cod Fatality

We have lost 12 Murray Cod in total with 2 more being found today. All dead fish are stored in the ARC Freezer for later examination and measurement. Water tests confirm that the water is healthy. It has been 4 weeks since the fish were introduced. The temperature of the recirculated system is 21.9C. Murray Cod prefer temps from 24-26C. We may have to move them from their current tank to an aquarium to better control the water temp?