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Oxygen Levels

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Maintaining Oxygen levels in Fish and KOI Ponds
Brett Fogle

Severe environmental stressors are the most frequent causes of
sudden fish death in garden ponds or other fish habitats, and the
lack of oxygen in the water is the number 1 stressor of all.

Two major factors contributing to fish-pond oxygen loss concern
blooming algae. During daylight hours, algae make oxygen; during
the night, however, the plants take oxygen. If algae are profusely
blooming, the plants deplete all the pond’s oxygen during the
night, which in turn, causes fish inhabiting the pond to die.

Another way that algae trigger oxygen loss is by dying suddenly.
Algae-bloom death occurs naturally or from chemical treatment of
an algaecide. When algae die suddenly – from either cause – the
oxygen becomes quickly depleted because the algae no longer are
alive and making it.

Then, when dead algae begin to decompose, the decaying process
even further depletes oxygen. Fish have been observed gulping for
oxygen at the water’s surface in sixty minutes or less from the
onset of sudden algae death.

In ponds stocked liberally with many fish, often the oxygen level
is just adequate enough to sustain them. If any further claim on
the existing oxygen is made, this delicate environmental balance
will be upset. When no or inadequate aeration is provided, along
with rising warm-weather water temperatures, the conditions for
fish survival become critical.

As water warms, its oxygen-retaining capacity decreases.
Consequently, small fountains, waterfalls, or other
water-aerating devices are incapable of providing adequate oxygen
for fish. Additional oxygen should be introduced through brisk
aeration when fighting algae, most importantly when water
temperatures surpass eighty degrees Fahrenheit.

Testing devices are the only sure method to accurately read
oxygen levels. Because these devices are not usually on hand for
the average person maintaining a water garden or fish pond, the
easiest way to ensure sufficient oxygen levels is to provide
generous and continual aeration. Pumps and diffusers that
“bubble” air into the water or large waterfalls, fountains, and
other “splashing” water devices are common ways to get sufficient
oxygen into the pond. When the weather is warm, or if an algae
problem exists, it is imperative that supplemental aeration be
provided constantly, throughout the day and night.

One other algae-caused loss of fish has gained attention and is
being studied in areas around the world. These fish-killing algal
varieties contain potent toxins that trigger sudden fish death.
If a pond hosts these particularly virulent algae, devastating
fish loss can result, seemingly more so when a quick die-off of
algal bloom occurs. The blooms discharge their toxins
concurrently, and some or all fish in the pond are adversely
affected or killed. Fortunately, since pond owners are unable to
identify these algae, this danger is rare.

Keep your oxygen levels under control at all times and you will
greatly increase your the lifespan of your fish.

Brett Fogle is the owner of MacArthur Water Gardens and several
pond-related websites including and He also publishes a free monthly
newsletter called PondStuff! with a reader circulation of over
9,000 pond owners. To sign up for the free newsletter and receive
a complimentary 'New Pond Owners Guide' for joining, just visit
MacArthur Water Gardens at

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