How Do You Check The Oxygen Level In A Koi Pond?

Since human beings cannot breathe underwater naturally, we are somehow programmed to think that all creatures living under water do not breathe.

Starting The Koi Hobby: A Step By Step Guide…

However, this is hardly true. Fish do breathe and need sufficient levels of oxygen to survive. That is why when constructing a pond, it is of the upmost importance to provide oxygen for koi fish and regularly check its levels.

Koi fish are a commonly preferred breed for decorative ponds. This symbolic Japanese fish is a beautiful addition to your pond’s aesthetics. Before inhabiting your pond with it, make sure you provide all necessary equipment for the fish’s survival.

So, how do you check the oxygen level in a koi pond? Buy a test kit. After that follow several steps:

1. Submerge the test kit’s water sample bottle completely, until there are not any air bubbles remaining inside.

2. Put on your protective gloves and glasses and open the water sample bottle. Add manganous sulfate.

3. Shake the water bottle gently to mix the chemicals.

4. Add sulfamic acid.

5. Fill the measuring tube with the water and chemical mix.

6. Add liquid sodium thiosulfate titrant.

7. Divide the number of sodium thiosulfate titrant drops you added by two.

Oxygen that consists in water is called dissolved oxygen (DO). Molecules from the air ‘fall’ into the water, dissolve and get adapted for respiration by aquatic life. The sufficient levels of oxygen are between 4 and 5mg per liter (milligrams per liter). Anything below this harms the koi fish and if the level drops below 2mg, the fish usually die. That is why it is so important to regularly check the oxygen levels in a koi pond.

How Do You Check the Oxygen Level in a Koi Pond?

Before you check the oxygen level in a koi pond, you need to do certain preparations and buy a test kit. The equipment you need for the test is:

  • Dissolved oxygen kit
  • Water sample bottle
  • Towel
  • Protective gloves
  • Safety glasses
  • Manganous sulfate
  • Alkaline iodide azide
  • Sulfamic acid
  • Measuring tube
  • Mixing bottle
  • Dropper
  • Sodium thiosulfate titrant

Below, you can find the different steps in details and what each one of them entails. Since many chemicals are involved, you need to have in mind a couple of safety precautions.

  1. Chemical agents are corrosive. Wear protective gloves and safety glasses when dealing with them and wash your hands thoroughly after finishing.
  2. If you are already treating your koi pond with chemicals, test kits might show false readings.

After you have taken all safety precautions, you can now proceed to checking the oxygen levels in your pond:

1. Take the test kit water sample bottle and submerge it completely until there are no air bubbles remaining inside. While under water, seal the bottle to prevent air from getting in. Take the bottle back to the surface and dry it with a towel.

2. Put on the protective gloves and safety glasses and open the sample water bottle. Add the manganous sulfate, followed by the alkaline iodide azide. Both are provided in the test kit, either in liquid or in powder form. If the chemical are in a liquid form, add 2ml of each. If like powder, add the full amount.

3. After you add the chemical compounds, seal the bottle again and gently mix. A brown substance called floc will appear. Let the stand still until the floc settles and later mix again for some additional floc formation. Repeat the process until the floc establishes itself fully.

4. Then, add the sulfamic acid to the mix. Same amounts apply, depending if it is in liquid or powder form. Shake the water again, after adding the new substance. The end result is the floc dissolving and the water turns yellow.

5. Fill the measuring tube from the kit with the water and chemical mix and pour it into the mixing bottle.

6. Gently swirl the sample in the mixing bottle with one hand while adding liquid sodium thiosulfate titrant at the same time. Use the dropper to add one drop at a time. Stop adding drops once the water turns from yellow to clear. Count the drops as you go. “

7. Finally, divide the number of sodium thiosulfate titrant drops you added by 2. This will give the koi pond’s dissolved oxygen levels milligrams per liter. Remember, the levels must not be below 4mg and definitely not below 2mg per liter.

Why Do You Have to Check the Oxygen Level in a Koi Pond?

Oxygen is, actually, one of the most neglected parameters in a koi pond. Low levels of dissolved oxygen are one of the most common reasons for fish deaths. If you regularly keep an eye on the healthy levels of dissolved oxygen in the pond, you will be rewarded by vibrant and energetic koi fish.

There can be various reasons why oxygen level drop, including atmospheric ones. Amongst the most common reasons are:

  • Atmospheric temperatures.
  • Weather conditions
  • Fish waste and decaying organic matter
  • Aerobic bacteria
  • Algae colonies and submerged plants
  • Warm water
  • Lack of sufficient water movement

Fish waste and decaying organic matter have a draining effect on the oxygen levels because a lot of oxygen is consumed during waste decomposition. Aerobic bacteria use a lot of oxygen to survive, same as algae and any submerged water plants. Also, warm water does not retain as much oxygen as cool water. When living in a warmer environment, koi fish are more active, consume more energy and food, and therefore require more oxygen.

In nature, fish receive their needed amount of oxygen through the natural movement of the water. For example: waves, streams, currents, water splashing into stones and shores, and from waterfalls. If you fail to provide enough water movement for your koi pond via air pumps, bubbles or artificial waterfalls, healthy oxygen levels will be immediately affected.

What are the Signs of Low Oxygen Level in a Koi Pond?

The sings can be multiple. For example, lethargic fish lacking energy; loss of appetite and general unhealthy look. If exposed for too long to low oxygen levels, the fish will eventually suffocate and die.

Can Koi Fish Survive if You Don’t Check the Oxygen Level?

No aquatic inhabitant can survive, if the levels of dissolved oxygen are very low. If you don’t check the oxygen level regularly, you risk leaving your fish to a random chance of survival. Even if you have provided an air pump, bubbler, waterfall and any other equipment that helps aeration, you can never be 100% certain of the oxygen levels. Equipment fails and you might not even notice. That is why it is important to check the oxygen availability in your koi pond regularly with a test kit.


To sum up, it is of the upmost importance to check the levels of dissolved oxygen in your koi pond regularly. This is done by using a test kit and following some well-defined steps. First of all, make sure you have protective equipment – gloves and safety glasses. Also, you need water from the pond, a bottle and a few chemicals contained in the test kit (manganous sulfate, sulfamic acid and alkaline iodide azide). The chemicals are found in either liquid or powder form. Mix the chemicals in the bottle and follow the exact proportions from the test kit. Mix until the water goes from yellow to clear. Finally, divide the number of drops that you have added from one of the chemicals by two and you will see the levels of dissolved oxygen in your pond. Remember, the DO (dissolved oxygen) must not be below 5mg per liter. If it drops below 2mg per liter, your koi fish will suffocate and die.

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