Is It Possible To Have Too Much Oxygen In A Fish Tank?

Is It Possible To Have Too Much Oxygen In A Fish Tank? 1

A volume of water can only hold a certain amount of oxygen. When a volume of water is holding the maximum amount of oxygen it can hold, it is said to be saturated. Once a volume of water is saturated, no more oxygen can dissolve into it. Oxygen can only dissolve into oxygen-saturated water in very special circumstances. This is what makes it very rare to have too much oxygen in a fish tank.

The water in your fish tank gets oxygen-saturated pretty often; you just do not know it. Oxygen-saturated water is totally not bad for your fish or any other oxygen-dependent tank inhabitant. This is because it is completely harmless. Your fish actually enjoys swimming in oxygen-saturated water just like you enjoy being out and about in nature.

So as I have stated above, oxygen-saturated is totally harmless to fish. However, as I have also stated, more oxygen can dissolve into saturated water in very special circumstances. When more oxygen dissolves into already oxygen-saturated water, the water becomes supersaturated. In other words, the water moves from a state of having maximum oxygen to a state of having too much oxygen.   

Unlike saturated water, supersaturated water is dangerous for fish.

What Are the Circumstances in Which Water Becomes Supersaturated?

Water can only become supersaturated in rare circumstances such as when a pump is cavitating or when there is a leaking water pipe. This is because such circumstances can increase pressure leading to more oxygen dissolving in water. 

If a fish tank is rapidly heated up by a heater or by being exposed to direct sunlight, the water in it can also become supersaturated because the quick temperature change can trap the excess dissolved oxygen that is usually present in cold water.

So there are only like a handful of circumstances that can result in there being too much oxygen in a fish tank.

What Happens When There is Too Much Oxygen in A Fish Tank?

Too much oxygen in a fish tank aka supersaturation is dangerous for fish. This is because when a fish breathes in supersaturated water, the excess oxygen will diffuse out of its bloodstream and create bubbles in its tissues. This effect is referred to as the gas bubble disease. It is pretty dangerous for fish. Sometimes it manifests as bubbles in eyes, fins, and gills. So it can make fish to look pretty horrific. In case the excess oxygen creates bubbles in a fish’s heart, it will probably kill it instantly. Simply put, supersaturated water is very dangerous for fish, unlike saturated water. 

How to Test Oxygen Levels

Now that you know there can be too much oxygen in a fish tank, the next thing to logically do is to frequently monitor the oxygen levels in your fish tank. This is because if you don’t, your tank may one day have too much oxygen that could actually kill your fish. 

There are three methods you can use to monitor the oxygen levels in your fish tank  

  1. Oxygen Test Kit

Oxygen test kits are a cheap and relatively reliable way to monitor the oxygen levels in your fish tank. They are cheaper than portable dissolved oxygen meters and more reliable than fish clues.

The kits are similar to the aquarium test kits used to measure nitrates, nitrites, and pH. So if you buy one, you will probably find it very familiar and convenient to use. However, you should make sure the test kit you buy and use is reliable as the many oxygen test kits available on the market have varying levels of reliability. 

  1. Portable Dissolved Oxygen Meter

There are many portable dissolved oxygen meters on the market. The very best ones are very effective in measuring oxygen levels in fish tanks. They can tell you exactly how much oxygen there is in your fish tank on a digital display.

However, they are usually pretty expensive compared to oxygen test kits. This is why people prefer oxygen test kits. However, if you can afford to buy one, you should buy it for it will certainly be more reliable and accurate than an oxygen test kit.

  1. Fish Clues

In case you do not have an oxygen test kit or a portable dissolved oxygen meter, you can monitor the oxygen levels in your fish tank by observing your fish for certain clues.  

Clues that indicate that there is excess oxygen in a fish tank include bubbles in the fins, gills, and eyes of the fish, while clues that indicate that there is very little oxygen in a fish tank include fish gasping for air and behaving erratically.

 While fish clues can help you to guess whether there is too much or very little oxygen, they are not reliable. This is because what you might think is a clue for too much oxygen or very little oxygen could be a clue for something else. 

How to Reduce Too Much Oxygen in a Fish Tank

In case you test the oxygen levels in your fish tank and discover that there is too much oxygen in your fish tank, you should act immediately to correct the situation. The only way to correct the situation of too much oxygen in a fish tank is to do a partial water change.

The assumption is that the water you will introduce will have less oxygen dissolved in it and will, therefore, reduce the concentration of dissolved oxygen in your fish tank to a harmless level. 

Follow the steps below to do a partial water change

  1. Treat tap water: Get a clean bucket and fill it with tap water and then add some water conditioner. This will treat the water, adjust its pH level, and eliminate chlorine which is dangerous for fish. Add only the recommended amount of conditioner for the volume of water in your bucket to be on the safe side.
  2. Switch off lights and everything else using electricity: Water and electricity do not go well together. So switch off lights and everything else using electricity to avoid getting shocked or damaging any equipment. 
  3. Scoop out water using a bucket: To shift your tank water from a state of supersaturation to saturation or non-saturation, you need to replace a significant amount of water. However, you should not replace too much water to avoid interfering with the natural balance of things in your tank. Therefore, aim to replace between 30 to 40 percent of the water in your tank. So use a bucket to scoop out this volume for replacement. And then scoop your fish with some water in a little bucket.
  4. Pour treated water into your tank: This is a pretty straightforward step. If the 30 percent or 40 percent of water you took out was 2 gallons, you are supposed to pour 2 gallons of the water you treated into your tank. 
  5. Measure the temperature of the mixed water in your tank: To avoid shocking your fish, you should measure the temperature of the mixed water in your tank and compared it with the temperature of the water in the little bucket where you kept your fish. If there is no significant difference, you should simply release your fish back into its tank. If the difference is significant, you should pour the water and your fish into a transparent polythene bag and immerse the bag into your tank. After about 30 or 40 minutes, the temperature of the water in the bag and in the tank will be the same. You should release your fish and take out the bag.

As you can see from the steps above, it is not very easy to change the water in your tank. However, it is necessary if you wanted to save your fish from supersaturated oxygen. 


400;”>Low oxygen in tanks is common because of things such as low lighting for plants, excess waste, lack of water movement, and overcrowding cause it. Too much oxygen is less common because it can only occur under very special circumstances. When it does occur, it can be very dangerous for fish because it can create bubbles in a fish’s tissues and even its heart. To prevent too much oxygen from harming or killing your fish, you need to reduce its concentration in your tank water by doing a significant water change.

Tammy Slater

Tammy Slater is the founder of, a home and garden blog that provides inspiration and resources for homeowners and renters alike. A self-taught DIYer, Tammy loves nothing more than tackling a new project in her own home. When she's not blogging or spending time with her family, you can usually find her rooting around in the garden or at the hardware store.

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