Ornamental Fish Q&A 24

An ornamental fish pond in a garden has been in operation for five years with no previous problems. It is heavily stocked with goldfish, tench, rudd, and a few large koi. There has been a recent period of hot, dry weather. Consequently, the water temperature is 5°C (9°F) warmer than usual. The water has turned green. The pond has a filtration system and aeration is supplied as a trickle of water circulating continuously, entering one corner of the pond. Early one morning, the owner finds that 25% of his largest fish are dead. The remaining fish are crowding at the surface of the water near to where the trickle of water enters the pond.

Question	Answer		Article
Why did the fish die?	The whole picture is one of low dissolved oxygen in the pond. The fish that died were asphyxiated. In hot weather, when the water temperature rises, the solubility of dissolved oxygen in water is lower. Phytoplankton thrive in sunny conditions; they remove carbon dioxide from the water and produce oxygen by photosynthesis, but they remove oxygen from the water at night as the respiration pathway takes place. With high feeding rates (and the fish would initially feed very well at higher water temperatures), more feces and waste enters the pond. Various bacteria break down this food (using up more oxygen). The amount of oxygen in the water will vary throughout the day. It will be lowest in the early morning before photosynthesis begins again. The fish are showing signs of oxygen deficiency by crowding around the area in the pond where there is likely to be the highest oxygen concentration.		[[|Link to Article]]
What is your immediate advice?	This is an emergency situation. Remember that the surviving fish will be severely stressed. The immediate recommendation is to provide adequate aeration. This can be done by agitating the surface of the water or attaching a spray device to the aeration equipment. Allow the water to spray up in the air to strip oxygen from the atmosphere before dropping back into the pond. Feeding should be severely reduced. It would be best to feed a few small meals in early- to mid-afternoon when photosynthesis is at its peak. Aeration should be continued throughout the night until the crisis is over. A dissolved oxygen meter would be very beneficial.		[[|Link to Article]]
What long-term advice can you give to prevent recurrence of the problem?	The long-term advice is based on explaining how oxygen depletion has killed his fish. The owner should be advised to assess the stocking density of the pond and be aware of the consequences of overstocking. After the problem has passed and the fish have returned to a normal, unstressed state, the pond should be cleaned out and water quality tests carried out on a regular basis. Cleaning out the pond will remove a lot of organic matter which consumes oxygen. It would be a good idea to have additional aeration equipment and a holding tank available in the future so that the fish could be split between the pond and the additional tank, thus reducing stocking density and improving oxygenation for the fish.		[[|Link to Article]]