Oxygen levels in a pond can dramatically change and cause the death of fish, some reasons can be environmental stress, but the main reason is lack of oxygen.
Two major reasons for oxygen depletion are the blooming of algae in the daylight which creates oxygen, however during the night the plant life uses it. If the algae is profusely blooming and creating oxygen then the plants take most of it during the dark, this depletes the oxygen and cause fish to die.
Another way for the algae to cause oxygen loss and fish to die is by the algae dying suddenly, when this happens it stops creating oxygen and the water again becomes depleted.
The algae starts to decompose and decay, this again depletes the oxygen levels, the process can be very quick, as little 1 hour sometimes between the decaying of algae, the depletion of oxygen and the death of pond life.
Remember aeration of a pond is very important especially when the temperatures increase during the warmer weather, more importantly, do not overstock your pond as it gets to a stage where there will not be enough oxygen to go round.
As water warms up its oxygen retaining capacity decreases, if you only have a small fountain or waterfall this may not create enough oxygen to sustain pond life, you will need to find another way to create more oxygen especially if the air temperature reaches 25 Celsius or above.
Remember, during the warmer weather aeration systems you have in place should run day and night to create enough oxygen, especially if you have an algae problem.
As long as you control your oxygen levels at all times, then the lifespan of your pond and fish will increase dramatically, it all seem a lot of work, but the pleasure you get from your pond and fish makes it all worthwhile.
Return from oxygen to water quality