The most frequent cause of death in pond fish are extreme environmental stressors such as the lack of oxygen. Blooming algae is the concern for the two of the major factors that contribute to oxygen loss in fish ponds. While algae makes oxygen during the daylight hours, plants take that oxygen back out of the water during the night. The death of pond fish can be brought about when the algae is profusely blooming and all the plants deplete all the pond's oxygen at night.
Algae can also trigger oxygen loss by dying suddenly, either naturally or by an algaecide chemical treatment. When this sudden death of the algae occurs, the algae are no longer able to make the necessary oxygen and therefore the level of oxygen is severely depleted. The oxygen in the water is depleted even further when the algae starts to decompose. Within one hour of the time that the algae dies, it has been observed that the fish in the pond often gulp for oxygen at the surface.
Oxygen in ponds that are liberally stocked with fish only provide for adequate amounts of oxygen to sustain them. The delicate environmental balance in this situation will easily become upset if any further claim on the existing oxygen is made. Conditions for fish survival often become critical when the warm-weather water temperatures rise and when there is little to no aeration provided for them.
When the water temperatures start to warm up, the water's capacity for retaining oxygen decreases. As a result, smaller waterfalls, fountains and other aerators cannot provide adequate oxygen for the fish to survive. When water temperatures surpass 80 degrees Fahrenheit, it is important that you introduce additional oxygen through brisk aeration, especially when you are fighting algae growth in the pond.
The only sure way of to reading the accuracy levels of oxygen in your pond is through a testing device. However, these devices are not available to the for the average person maintaining a backyard fish pond or water garden. The easiest way to ensure proper oxygen levels is through continual and generous aeration through pumps, diffusers, and airstones that bubble air into water as well as other splashing devices. Larger fountains and waterfalls will provide more aeration and oxygen for the pond. Be sure that supplemental aeration is constantly provided, day and night when an algae problem exists, or when the weather gets warm.
Another cause of sudden death in fish is a variety of algae that contain potent toxins. the death resulting from this variety of algae often causes a more devastating result than the a quick die-off of algal bloom. This is because of the concurrent discharge of toxins into the water; resulting in the deaths of some or all of the fish. Fortunately, the danger is rare since pond owners are unable to identify these algae.
You can increase the life span of your fish and make them happy and healthy by keeping your oxygen levels under control at all times.