You might not be able to see it, but it’s actually one of the most important parts of your aquarium.
No, I’m not talking about the canister filter hidden away underneath your aquarium.
I’m talking about oxygen.
It’s funny – one of the only times you will think about the oxygen in your aquarium is when it isn’t there.
And by then it’s too late.
So sit back as I teach you everything you need to know about this important aquarium ingredient.
Where does the oxygen in your aquarium come from?
Even though H2O (Water) is part oxygen, it is bonded with hydrogen – essentially locked together, thus making it inseparable.
Your fish can’t breath this.
So if that’s the case, then where does the oxygen that your fish breath come from?
Believe it or not, plants and fish actually breath the exact same oxygen as you and I.
In order for your fish to “breath” oxygen, it needs to get inside the water. And how it gets there is actually very interesting…
The surface of your aquarium is always in contact with the air. And it is here, at the surface, that oxygen enters the water through a process known as gas exchange.
The reason it is called gas exchange is because the air and water do a trade. The water in your aquarium swaps carbon dioxide (CO2) for the oxygen (O2) in the air.
I find that a diagram makes it much easier to understand…
Pretty simple, huh?
Since fish breath in this oxygen and give off carbon dioxide, this exchange is a continuous process.
It is also worth mentioning that the larger the surface area of the water, the more efficient the gas exchange.
So a larger tank is much easier to oxygenate than a small one.
Now you may be wondering:
How much oxygen does your fish tank need?
Unfortunately, this is a difficult question without a clear answer…
Especially given that there are different factors contributing to how much oxygen can actually dissolve in your fish tank, including:
1. Water temperature – As the water temperature increases, the amount of oxygen that water can hold decreases. Tropical fish have less oxygen in their water than cold water fish.
2. Salinity – To put it simply, salinity is the measure of the amount of dissolved salts in your water. The saltier your water, the less oxygen it can hold. You can test the salinity of your aquarium with a refractometer.
3. Atmospheric pressure – The lower the air pressure, the less oxygen the water can hold. It may surprise you to learn that, all thing being equal, water in Miami (altitude 6 feet) can hold more oxygen than water in Denver (altitude 1 mile).
Then there’s the fish…
Generally speaking, larger fish need more oxygen than smaller fish, while slower moving fish need less oxygen than fast swimmers.
So, as you can see – the answer to how much oxygen your aquarium needs isn’t exactly clear cut.
That said, there are recommendations as to the ideal oxygen levels for certain aquariums, measured in Parts Per Million (PPM).
|Freshwater Fish||8.3 PPM|
|Marine Fish||Between 7.0 and 6.4 PPM|
Please note that these are only loose guidelines and the previous factors I listed will impact just how much oxygen can dissolve in your tank.
But if your fish are behaving normally, and are not gasping for air at the surface of your aquarium, then it is a good sign that there is enough oxygen in your aquarium.
In fact, fish behavior and oxygen levels go hand in hand.
Which brings me to my next point…
How do you know if your aquarium is low on oxygen?
Above, you can see a picture of water that is high in oxygen.
The next picture shows water with dangerously low levels of oxygen…
Notice anything different?
I would hope not!
I don’t care how good your eyesight is, it is practically impossible to determine if water has an oxygen shortage just by looking at it.
While you could test your water for dissolved oxygen, which I will get to in a moment, there is one other way to determine if your aquarium has low oxygen levels…
By looking at your fish.
Your fish will behave differently according to just how much oxygen is in the tank.
The first sign that you have an oxygen problem is slower movement. At the first sign of low oxygen levels, fish will slow down and swim less. This is particularly noticeable in active fish.
As oxygen levels continue to drop, you will notice that your fish has trouble “breathing.”
If you look closely at the gills, you will notice them moving rapidly as your fish desperately attempts to get enough oxygen by passing more water than usual through their gills.
If the oxygen levels are drastically low, then the symptoms become obvious – your fish will gasp for air at the top of your aquarium.
Your fish will look noticeably distressed as they attempt to draw oxygen from the most oxygen-rich location in your aquarium, the surface layer.
If your tank is home to many different types of fish, you may notice that only a few gasp at the surface for air.
You should still be concerned, as those that are not gasping for air are probably stronger or require less oxygen – they too will eventually be affected by low oxygen levels if the problem is not corrected.
Fish show these obvious signs of stress when the oxygen content of water drops below 4 PPM. If the oxygen drops below 2 PPM, death will soon follow.
So now that you know how to identify the problem, it’s time to determine the cause…
What causes low oxygen in aquariums?
By far the most common cause of low oxygen levels in aquariums is overstocking – keeping more fish than is appropriate for your tank.
Each fish that you add to your tank needs oxygen to breath. And once you add too many fish, the oxygen in the water will be consumed at a faster rate than it can be replenished.
And the consequence is that there isn’t enough oxygen in the tank for your fish to breath.
Fortunately, the solution to overstocking is simple:
There are calculators available online that can help assist you with how many fish you should stock in your tank.
Compared to overstocking a fish tank, the remaining contributors to declining oxygen are quite minor. However, when combined with an overstocked aquarium, these factors can be dangerous.
I know, it seems like a funny one…
Living plants use carbon dioxide (CO2) and give off oxygen – so how can they contribute to lower oxygen levels?
Well, here’s the thing:
Plants only give off oxygen when exposed to light, whether that’s natural daylight or the light from your LED setup.
When night falls, and the tank turns dark, the opposite happens – plants consume oxygen and give off CO2.
If your planted aquarium has reduced or no light for an extended period of time, the plants could deplete enough oxygen to affect your fish, especially in an overstocked tank!
Note: Algae is also a plant, and this problem can also occur in tanks with a heavy algae bloom.
3. High Water Temperature
As I touched on earlier in this article, warm water cannot hold as much oxygen as cool water.
In summer, the water temperature of your aquarium can spike – especially if it sits in direct sunlight, like near a window!
And when this happens, there is going to be less oxygen in the water for your fish.
If you notice that your aquarium temperature is higher, then you have a few options available:
- Shade the aquarium from sunlight
- Perform a water change with cooler water
- Turn off the heaters until temperature returns to normal
- Use a fan or aquarium chiller to blow cool air over the surface
- Float a zip-close bag with ice cubes inside in your aquarium
4. Excess waste
Okay, so excess waste isn’t directly responsible for a lack of oxygen in your aquarium…
But it does kick off a chain of events that can lead to low oxygen levels, so I felt that it should be included in this list.
Fish poop, plants decay and uneaten fish food will rot…
Bacteria that feed on these waste products also require oxygen, further depleting the oxygen levels in your tank.
Excess waste can also lead to an algae bloom, which can also lead to oxygen loss.
Oh, and if that’s not enough, too much ammonia from waste can irritate your fish’s gills, making it more difficult to draw oxygen out of the water.
Long story short – don’t let waste build up in your tank!
When using chemical additives, it is recommended you always read the instructions to determine if they will cause any negative effect, like lower oxygen levels.
How do you test the oxygen levels of your aquarium?
For hobbyists, there are two common devices used to measure the amount of dissolved oxygen in an aquarium
Let’s start with the cheapest option…
1. Dissolved Oxygen Test Kit
If you have ever tested the pH of your aquarium, you will be all too familiar with colorimetric tests.
Well, aquarium test kits work in a similar way, and can even be used to check the oxygen levels in your aquarium.
Simply add the drops to a sample of your aquarium water and compare the color to the chart to determine the amount of dissolved oxygen in your tank.
Generally speaking, the more expensive the test kit, the more accurate the reading will be. That said, being precise isn’t too important to hobbyists, who will generally only need a rough idea of their dissolved oxygen levels.
Each dissolved oxygen test kit is capable of performing multiple tests. The Salifert test, pictured above, is capable of performing over 40 tests.
2. Dissolved Oxygen Meter/Probe
If you have the cash to spare, a dissolved oxygen meter precisely measures your aquarium in just a few seconds.
Stick the probe in your aquarium and the amount of oxygen in your aquarium will be displayed on the LCD screen.
While a dissolved oxygen meter may be more accurate than a test kit, it also requires continual maintenance to ensure the results remain accurate. Membranes and batteries must be replaced and the meter must be calibrated regularly.
But if you want the most accurate measurement possible, You can’t go past a dissolved oxygen meter.
How to increase the oxygen levels in your aquarium
Well, the first thing you want to do is remove all the contributing factors to oxygen depletion that I mentioned earlier, namely overstocking your tank.
With that out of the way, there is actually a clever trick you can use to increase the amount of oxygen in your tank.
I mentioned earlier how a larger tank exchanges gas more efficiently than a smaller tank due to the larger surface area of the water.
Well, there is actually a way you can “artificially” increase the surface area of your aquarium.
And that method is surface agitation.
To put it simply, water movement on the surface of your aquarium increases its surface area, allowing more oxygen to dissolve and more carbon dioxide to escape.
Any product that helps produce water flow is perfect for agitating the surface of your aquarium, including:
- Aquarium bubbler (like an airstone)
- HOB filter, or point your filter return at the surface
- Spray bar
- Lily pipe
By agitating the surface water in your aquarium, you can potentially stock more fish than you otherwise would have been able to without it.
How to increase oxygen levels in an emergency
So you have just come to discover that panicked fish are gasping for air at the surface of the tank.
There is no time to look for the cause of the problem, you can do that when your fish are safe.
Right now, your priority is saving your fish.
The easiest and quickest way to get oxygen into your aquarium is to immediately perform a large water change – as much as 50%.
The new water will bring with it dissolved oxygen that should keep your fish happy in the short term.
This will buy you some time so that you can hunt for the cause of the depleted oxygen.
Too much oxygen in an aquarium…
A reader actually asked a rather interesting question, one that I thought would make a great addition to this guide…
What happens if there is too much oxygen in an aquarium?
Water can only hold so much oxygen before it reaches saturation.
And once saturation is achieved, no more oxygen is going to dissolve in the water.
Your fish will happily breath in water that is saturated with oxygen – it’s harmless.
But the problem comes when water is supersaturated with oxygen.
And I must stress that this isn’t common in an aquarium environment.
One way that tank water can become oversaturated is from a leaking water pipe or cavitating pumps that increase pressure, resulting in excess oxygen to dissolve in the water.
The other way is through rapid heating of your aquarium. Because cold water can contain more oxygen than warm water, the oxygen gets trapped due to the extremely fast temperature change.
When your fish breaths in the over-oxygenated water, the oxygen leaches out of your fish’s blood stream and forms bubbles in the tissues.
This is known as gas bubble disease and it isn’t pretty…
Visible bubbles can form in the gills, fins and eyes of the fish. Gas bubbles can also build up in your fish’s heart, leading to death.
It may be invisible, but there is no denying that oxygen plays a hugely important role in your aquarium.
Do you regularly test the oxygen levels in your aquarium? Let me know in the comments below!
Ian Sterling, founder of Fishlab.com, began his aquarium journey over 30 years ago, driven by a deep fascination for fish and their diverse personalities. His website, Fishlab.com, is dedicated to making fishkeeping accessible and enjoyable, offering beginner-friendly guidance, expert insights, and a community for aquarists to connect and share experiences.