While it s thankfully not a common occurence, some hobbyists have lost whole ponds full of fish overnight under extreme conditions. The situation can become even more critical if the fish are overfed, or if there is a great deal of decaying debris in the pond, since the microscopic organisms that decompose waste also consume oxygen. Some fish medications, especially those that contain formalin, also chemically remove significent amounts of oxygen from solution. It s commonly understood that all live plants consume carbon dioxide and actually produce oxygen via photosynthesis, but that only occurs during the daylight hours. At night, when there is no energy from sunlight, plants consume oxygen just as animals do .
Extra pumps are highly recommended.
Excessive amounts of debris and algae should be removed. Remove uneaten food promptly. Thin out the fish population if necessary. It s far better to have fewer, but healthier fish. Small, frequent water changes can reduce the amount of chemicals and microorganisms that consume oxygen.