Fish need oxygen

That is an understatement. Oxygen is one of the most overlooked parameters of water quality in the koi hobby. Low dissolved oxygen level is an underestimated cause of many fish losses. As a koi keeper it is imperative that you regularly test your pond water and monitor its dissolved oxygen level. Knowing oxygen requirements for optimum fish health will go a long way in your desire to keep and grow healthy koi.

What makes the oxygen level drop in a koi pond, you might ask. Many factors affect the dissolved oxygen concentration in pond water. It’s not just the fish load in your pond. Atmospheric temperatures as well as weather conditions affect dissolved oxygen. Fish waste and decaying organic matter have a draining effect on dissolved oxygen levels because oxygen is used up during decomposition.  Aerobic bacteria (good bacteria) consume a lot of oxygen. Algae blooms and submerged plants also have an adverse effect on oxygen level because they consume a lot of oxygen during the nighttime hours, through photosynthesis. This is why you may discover that the dissolved oxygen concentration of your pond is quite low in the early morning hours.  If the demand for oxygen by plants, bacteria and fish is greater than the dissolved oxygen level of the pond, your fish are most likely to suffer.

Warm water does not carry as much oxygen as cool water and fish require more oxygen in higher temperatures as they are extremely active and eating more often. In cool water fish are less active and their oxygen requirement is minimal.

Dissolved Oxygen Chart

The minimum dissolved oxygen concentration for healthy growth, tissue repair and reproduction in koi is 6 mg per liter of water, but ideally the concentration should be higher. An oxygen level over 7 is desirable, and any reading above that should be considered excellent.

The minimum dissolved oxygen concentration for healthy growth, tissue repair and reproduction in koi is 6 mg per liter of water, but ideally the concentration should be higher. An oxygen level over 7 is desirable, and any reading above that should be considered excellent. Generally, prolonged exposure to low dissolved oxygen is harmful to all fish. A habitually low oxygen level can be linked to slow fish growth, ersistent fish sickness and may contribute to repetitive parasite outbreaks. Continued exposure to low dissolved oxygen is also considered a precursor to bacterial infections in fish. Please note that tap water and well water have frighteningly LOW levels of dissolved oxygen. You should pay close attention to the oxygen levels whenever a water change is being performed.

What are the signs of low oxygen level in a koi pond?

Fish may congregate near an oxygen source such as waterfall or bubbler when the dissolved oxygen level drops below to an uncomfortable level. In extreme cases of low oxygen, 3-4 mg/L, fish will gasp or pipe at the pond surface and may die after an extended period of low oxygen concentration. Large fish will suffer and die first during an oxygen crisis because they have a higher oxygen demand to live.

What is a good oxygen level for a koi pond?

A dissolved oxygen level in the range of 7-9 mg/L is ideal for koi and goldfish to live and thrive.

Do waterfalls supply oxygen for my fish?

Yes, a waterfall is beneficial and can help to replenish the oxygen in a contained pond. However, for ponds with a depth over 48” a supplemental aeration pump with diffuser stone is recommended. The diffuser, or air stone, should be submerged in the deepest portion of the pond and rest on the pond bottom.