When you see your fish gasping for air at the surface of your pond immediate action is required. Your fish are not trying to say hello, they are trying to tell you that they are in survival mode. The reason they are at the surface trying to breathe is due to a lack of dissolved oxygen in the water.
The low levels of oxygen are mainly due to a lack of aeration or poor water quality, which can then cause the fish to stress. This stress slows their immune systems and leaves them vulnerable to parasites and disease. But you can avoid this by aerating the water and keeping the water quality crystal clear.
Fish need different amounts of oxygen depending on the season. In winter they need less because they are hibernating but during summer months, when they are most active, they demand a higher amount. With warmer temperatures, water holds less dissolved oxygen leaving aquatic life competing for the little that remains. This is why supplemental aeration is so important. You can add pond plants, which naturally release oxygen into the water during the daytime. You should also keep your pond pumps and fountains running 24/7. You can also add an aerator, a waterfall, or spitter to make sure the oxygen levels are sufficient.
Poor water quality can also attribute to fish gasping for air at the pond’s surface, so you should also check your filtration system and use a test kit to test levels of ammonia and nitrite. Using pond cleaner tablets will add beneficial bacteria and a water clarifier and chlorine remover will help to ensure excellent water quality for healthy fish.
If your fish are gasping for air you should quickly oxygenate the water by turning on a hose or any other water pump and spraying the water in the air so it gets oxygenated and then lands in the water garden. Doing a partial water change is also recommended. If it is chlorinated water and you don’t have chlorine remover handy then only do a 10% water change. If the water has more than a 10-degree temperature difference then do the water change over several hours.
Once you have solved the crisis, try to figure out what caused the sudden loss of oxygen. Often times depleted oxygen levels are due to excessive algae blooms, dead organics, excess nutrients from storm water runoff, and fish disease/parasite. Checking the pond site and using a test kit will help to determine the cause as well as determine the steps to take to fix the problem in your pond.