Does My Pond Water Have Enough Oxygen? (How to Test)


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How To Test Oxygen Levels In Pond Water & Detect Signs of Low Oxygen

A group of koi fish swimming in a pond
Koi ponds should have dissolved oxygen concentrations of at least 8 ppm. Eddie Maloney from North Las Vegas, USA, CC BY-SA 2.0, via Wikimedia Commons

Just as we need oxygen to live, fish and other pond inhabitants require adequate oxygen levels to respire. Even bacteria, fish and insect eggs, larvae, algae, and zooplankton need access to this essential element. Oxygen is vital in nutrient cycling and in the breakdown of organic matter. Without it, a pond ecosystem would quickly collapse and could become toxic to animal and plant life.

In ponds, oxygen is present in its dissolved form. It enters the pond system in many ways – by diffusing through the water’s surface, via wave and wind action, and through photosynthesis. Productive ponds, with high biomass, can quickly deplete dissolved oxygen levels in the absence of a mechanized pump or aerator. This can lead to oxygen stress, a scenario that any responsible pond owner must avoid as it can quickly end in fish deaths.

Fish ponds should have dissolved oxygen (DO) concentrations that are consistently above 6 ppm, going up to 8 ppm for koi. This optimal level increases the chances of rapid fish growth and high survival rates. Every pond owner must learn how to identify early signs of oxygen depletion, which can still be reversible. Luckily, when it comes to low oxygen, ponds communicate using cues that are readily noticed by the observant eye.


Signs of Low Oxygen in Ponds

When a pond has enough oxygen, fish remain active and should eagerly consume food whenever they are fed. They will venture to various levels of the water column, exploring with ease and curiosity. A fully oxygenated pond should remain clean and clear. It should generally be pleasant to be around and should not display any of these dangerous signs:

1) Algal blooms

Algal blooms in a pond
Algae can quickly multiply and form dense mats, using up any remaining oxygen molecules in a pond. tuchodi / CC BY-SA 2.0

Oxygen is a requirement for carbon and nitrogen cycling processes (aided by the action of nutrient cycling bacteria) which are responsible for breaking down fish waste and converting harmful compounds into their harmless counterparts. Depleted oxygen concentrations result in higher levels of ammonia, nitrite, and free-floating organic compounds.

These are used by algae to quickly multiply, forming dense colonies. These can be so thick that they form mats on pond surfaces, blocking out light, competing with pond plants, and using up any remaining oxygen molecules.


2) Fish gasping for air

Koi with gaping mouths at the pond surface
Fish congregating at the water’s surface with gaping mouths outside of feeding times is an early sign of low dissolved oxygen. Mark Doliner / CC BY-SA 2.0

An early sign of low dissolved oxygen is fish congregating at the water’s surface with gaping mouths. This can easily be mistaken as hunger, but is surely a struggle for survival when it occurs outside of feeding times. Fish gathering close to the outflow point of a filter can also indicate low DO before a collapse occurs. At an advanced stage of oxygen depletion, they can display odd swimming behavior or remain immobile on the pond bottom. This type of stress can cause irreversible damage to their organ systems. 


3) Foul odors

A natural pond
Natural ponds often smell foul due to a high amount of decaying material and a lack of aerators. Photo from pxfuel

When oxygen is no longer available, the breakdown of organic materials becomes controlled by anaerobic processes or fermentation. This is usually accompanied by putrid or foul odors, similar to those that would be emitted by rotten plants and animals. Natural pond and lake systems can often smell foul due to the high amount of decaying material and the absence of aerators to replenish depleted oxygen levels, especially in isolated pools.


Causes and Dangers of Low Oxygen

Pond pump by the pond edge
Low oxygen in ponds can be caused by a lack of pumps relative to the water volume. osseous / CC BY 2.0

Common causes of inadequate DO levels in fish ponds include overcrowding, overheating, a lack of pumps or aerators relative to water volume, absence of oxygenating pond plants, absence of light (as a requirement for photosynthesis), and the presence of decaying materials. Excess poor-quality feeds, an inefficient pond filter, and damaged plants can increase the nutrient load of your pond, placing heavier demands on oxygen. The entry of runoff, fertilizer, and natural waste from other sources can also significantly use up oxygen.

Experienced pond owners will seldom come across these issues due to proper pond planning and keen observation. It can be tempting to overstock an ornamental pond and keep it free of submerged and marginal plants to increase surface visibility. Unless your pond is equipped with the mechanisms to tightly control parameters of intensive fish culture, it can struggle to maintain a healthy environment for your fish. Keep in mind that oxygen production should always be higher than oxygen demand.

Ornamental fish are extremely sensitive to drops in oxygen, and may be the first to noticeably become compromised. Their immune systems can weaken due to hypoxia, making them susceptible to parasites and diseases. They are also less likely to recover from periods of sub-optimal temperature conditions and high-energy activities (such as spawning and acclimation) when DO levels are below their recommended range. Top-value, facility-bred species tend to be accustomed to DO levels that range between 7 – 8 ppm.


Measuring Oxygen

A dissolved oxygen testing kit
Testing for DO levels using a meter or test kit should be a part of routine pond maintenance. Glen Bledsoe / CC BY 2.0

Breeders and aquaculture pond technicians rigorously measure pond DO levels, sometimes even 3 times during the day and once at night. They must do so because a slight drop in oxygen can result in lower quality fish and the death of a few individuals, translating into heavy financial losses. Depending on your pond features and personal goals, the frequency rate at which you measure DO levels will differ.

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If you have an ornamental koi pond and it is stocked to maximum capacity, it would be prudent to take measurements on a daily basis. If you have a fully naturalized or wildlife pond, periodic tests taken every few days may suffice. Testing for DO levels should always be a part of routine pond maintenance. If you are able to access a nearby laboratory, they should be able to give you more accurate results. If not, a digital DO meter or a test kit should do. Do note, however, that these are not always accurate and should be assessed alongside your own observations.

Water samples should be collected from a region of the pond that best represents general conditions. The sample bottle should be filled at a median depth and should not contain bubbles. Cap it once it has become filled to the brim with water. Samples should not be exposed to sunlight and should be refrigerated if you are unable to test them right away. If using a test kit, follow the instructions found on the label.


How to Increase Oxygen Levels

A pond filter
Cleaning your pond filter is one way to increase oxygen levels in your pond. Carolyn from Pemberton Township, NJ, USA, CC BY 2.0, via Wikimedia Commons

Below is a brief list of natural and artificial means of increasing oxygen levels. For best results, a combination of these can be used. Always try to find the root cause of oxygen depletion and make sure to resolve issues prior to investing in more equipment or accessories.

  • Add an aerator or pond pump. If you already have these (which you should!), make sure to maintain them properly. You can experiment by keeping them on for longer periods of time or by adding more to your pond setup. A mature pond may require more aerators than a new one. Always keep a spare pump in case of emergencies.
  • Clean your pond filter. The more waste it can accumulate, the better. Less waste in the pond reduces the demand for oxygen.
  • Perform regular partial water changes.
  • Add submerged and marginal plants to your pond.
  • Remove any decaying plant material. You can make use of a net to gather debris that has settled to the pond bottom or is floating at the surface.
  • Provide partial shade. Cooler pond water tends to have higher DO concentrations because oxygen is more soluble at low temperatures.
  • Add a water feature that agitates the surface of your pond. A mini fountain, waterfall, or pond spitter can significantly increase DO levels and keep the water temperature cool.
  • Emergency DO source: a low-concentration hydrogen peroxide solution may be a quick and temporary fix to dangerously low DO levels, giving you a couple of hours to come up with a more permanent fix.

Pond Oxygen in Winter

A frozen pond in a garden
If your pond freezes over, you should create a hole in the ice to help oxygen diffuse into the pond. Tejvan Pettinger / CC BY 2.0

Winter conditions can be problematic for the maintenance of DO levels in outdoor ponds even if cold water has a higher capacity to “hold” oxygen. If the surface of your pond freezes, you will have to de-ice a section of the pond or create a hole in the ice. An open area is essential as oxygen will need to diffuse into the pond through the water’s surface. DO levels will still need to be monitored regularly at this time, especially if you have fish in the pond. If oxygen levels are optimal, fish will likely keep to the warmer, deeper areas of the pond.


Can a Pond Have Too Much Oxygen?

Bubbles on the water's surface
Tiny bubbles on the pond’s surface may indicate excess DO levels. Michael Knapek / CC BY 2.0

An excess of oxygen rarely occurs in ornamental or wildlife ponds. This condition is called supersaturation, and is normally a result of high-pressure oxygen input from fine boreholes (such as from a perforated pipe). This can be dangerous to fish and cause them to quickly deteriorate.

Supersaturation can lead to gas bubble disease, which is comparable to the “bends” in scuba divers that surface too quickly. Swirls of bubbles surfacing from the pond bottom or tiny bubbles dotting pond surfaces can indicate an excess in DO. As always, regular maintenance and close observation are key to avoiding this phenomenon.

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