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Testing oxygen (O2) in a fish tank is simple! While a dissolved oxygen (DO) meter may be slightly pricier than a dissolved oxygen test kit, they are the most accurate way to measure oxygen levels in a fish tank. A DO meter includes an oxygen probe that assesses the amount of DO in the water.
Oxygen (O2) is a molecule that is essential for all life, including fish. The dissolved form of oxygen is referred to as dissolved oxygen (DO). Fish must absorb DO directly from water, so it can enter the bloodstream and pass through their gills for respiration. While there are some species of fish (like gouramis and betta fish) that can breathe O2, sufficient DO levels must be maintained in water for all fish.
The reefing hobby is constantly growing in the United States (and worldwide!), with over an estimated 8 million indoor fish tanks (17% of the US population) in 2022, compared to 5 million fish tanks (12% of the US population) in 2021.
Like us humans, fish do not produce oxygen. Fish do however need oxygen to survive. Although your fish are submerged in water (H2O), which is part oxygen, they are unable to utilize it in this form because the oxygen molecule is tightly bonded with hydrogen, practically locking it together. Unfortunately, fish in a fish tank cannot breathe this.
So, where do fish in a fish tank get oxygen from?
You may be surprised that the fish in your fish tank breathe the same oxygen as humans. This means that it must enter the water inside your fish tank.
The surface of the fish tank water is always in contact with air, which contains oxygen. So, if you haven’t worked it out yet, this is where the oxygen enters the water. This process is called gas exchange and can happen over any surface of water. During the gas exchange, the fish tank water exchanges carbon dioxide (CO2) for O2 in the surrounding air.
As fish give off CO2 as a waste product like we do, this gas exchange process is continuous. We should also mention that larger fish tanks will have a greater surface area, and therefore more efficient gas exchange.
Once the oxygen has entered the fish tank water, it is referred to as dissolved oxygen (DO).
Dissolved oxygen can also be supplemented in a fish tank via an aeration system. This typically consists of an air pump, airstone, and tube. This system will not increase the O2 level in the fish tank significantly, however, it can help in circulating O2 around the water and break the surface tension to speed up the rate of diffusion.
Plants are another fantastic way to increase O2 levels inside a fish tank, plus they create a stunning aquarium. Plants naturally produce O2 as a by-product of photosynthesis in the presence of light. When no light is available in a fish tank, plants and fish will continue to consume O2 and produce CO2 but no O2 is produced, therefore light plays a key role in a fish tank’s setup.
The amount of oxygen needed in a fish tank depends on how many fish are stocked in the aquarium, the presence of plants, and the species of fish. However, typically all fish tanks should have a concentration of 5mg/L of DO.
Many factors contribute to how much O2 can dissolve in a fish tank:
Water Temperature
Warmer water holds less O2/DO than colder water. For example, tropical fish tanks will generally have less O2 than cold-water fish tanks.
Salinity
Salinity measures the amount of dissolved salts in aquarium water. So, the more salt in the water, the less O2 it can hold. Salinity is an easy water parameter to measure – a refractometer accurately measures salinity in water and they are inexpensive.
Atmospheric Pressure
The higher the air pressure, the more O2 the water can hold, and vice versa.
Fish
Typically, if you have larger fish they will need more O2 than smaller fish. Also, faster-moving fish will need more O2 than slower-swimming fish.
These are just a general guide of dissolved oxygen in water for fish – always refer to the O2 levels for your species of fish and monitor their behavior. For example, if they are gasping for air at the water’s surface, likely, the fish do not have enough O2 in the fish tank.
Oxygen content cannot be determined by looking at the water, however, there are some signs that your fish and plants may exhibit to warn you that the O2 level is too low.
Fish are good indicators of low O2 levels in water because they will behave differently. For example, the first sign of an O2 issue is slow movement. When O2 levels drop, fish will slim slower and less to conserve O2 levels across their gills. So, if you have very active fish such as tangs, sudden slow movement is usually the first sign you need to increase O2 in the fish tank.
If O2 levels continue to decrease and become too low the aquarium will become hypoxic, and therefore fish will start to struggle breathing. If you take a close look at your fish you will notice that their gills are moving rapidly. They do this in an attempt to desperately absorb O2 by passing more water over their gills.
If the O2 levels deplete even more, the fish will resort to gasping for air at the surface of the water and will be noticeably distressed.
If you have many fish inside your fish tank but only a handful are gasping at the surface, you should still be concerned as the ones not gasping will eventually be affected by O2 depletion inside the fish tank.
In most cases, fish will start showing signs of stress when the O2 level is below 4 ppm. If you don’t do anything and allow the O2 level to drop below 2 ppm the fish will shortly die from suffocation.
We should also mention that some fish like gouramis and bettas will gulp air from the water’s surface. Do not worry too much as this is normal behavior, however, if they remain at the surface gasping for air for a long time and with their mouths wide open, test the O2 level inside your fish tank.
The most common reason behind low O2 levels in a fish tank is overstocking. Overstocking can easily happen as hobbyists get too excited and buy too many fish for the size of the aquarium at home.
Remember that every fish you put inside your fish tank must be able to breathe. So, if you add too many fish, the O2 in the water will be consumed at a much faster rate than it can be refilled.
The next points are minor compared to overstocking, however, when combined, it can cause major issues with the amount of oxygen levels in your fish tank.
Luckily, there are two fantastic ways to test oxygen levels in fish tanks!
The cheapest option is a dissolved oxygen test kit.
For those on a budget, an aquarium test kit that tests the oxygen levels inside your fish tank is a great start.
How to use a dissolved oxygen test kit:
There are many different types of DO water test kits, and typically, the more expensive the kit, the more reliable the results will be. However, if you want super reliable readings it is worth investing in a dissolved oxygen meter/probe.
Dissolved oxygen meters are a great way to precisely measure the O2 level in a fish tank, plus they are very easy to use:
At Atlas Scientific, we have a range of DO probes to test dissolved oxygen in water. For fish tanks, we recommend the Atlas Scientific EZO-Dissolved Oxygen Kit as it gives you everything you need to take highly accurate and interference-free readings, plus it can be submerged in salt or freshwater aquariums.
Another way to test oxygen levels in fish tanks is with an oxygen saturation probe. This is another electronic device that can measure DO in water, but instead of directly measuring the O2 content, it detects the oxygen saturation of the water.
If your results indicate low O2 levels in the water, you will want to increase the O2 level as quickly as possible.
First, you should remove contributor factors that have caused the O2 levels to drop, such as excess waste or too many fish.
Next, you want to artificially increase the surface area of the aquarium water via surface agitation. This simply involves moving the surface of the water allowing more O2 to dissolve and CO2 to escape. You can use a powerhead, wavemaker, spray bar, aquarium bubbler/airstone, or HOB filter.
In emergencies where fish are gasping drastically at the surface, you may want to skip the possible causes and jump to saving your fish as quickly as possible.
The fastest way to get oxygen into your fish tank is to perform a large water change – up to 50%! This new water should bring DO into the water, making your fish more comfortable in the short term. This isn’t a permanent solution, but it will buy you some time to identify what went wrong and how you can prevent O2 levels from dropping again.
Oxygen is a requirement in a fish tank. All fish require oxygen to survive and therefore understanding the signs of oxygen depletion and how to test oxygen levels is a must. The most accurate way to measure oxygen levels in a fish tank is a dissolved oxygen meter/probe.
If you are unsure which dissolved oxygen meter will suit your testing needs, do not hesitate to contact the world-class team at Atlas Scientific.
Dissolved oxygen measurement is essential for monitoring water quality in various applications. It’s used in wastewater treatment to ensure optimal conditions for bacteria, in aquaculture
Dissolved oxygen (DO) plays a crucial role in groundwater quality. It supports bacteria that break down pollutants and minimizes harmful substances like iron and manganese.
