Can A Fish Tank Be Too Oxygenated?

One of the biggest concerns for fish tank owners is low oxygenation. Too little dissolved oxygen will stress the fish and can kill them. But what about over oxygenation? Can a fish tank have too much dissolved oxygen? 

That’s what we discuss in this article, including the dangers of too much oxygen in a fish tank and how to prevent it.

Importance of Adequate Oxygen in a Fish Tank

Fish, like other animals and humans, need oxygen to survive. And since they, with a few rare exceptions, cannot breathe air like land dwellers, they rely on oxygen that is dissolved in water.

That is why it’s important to monitor oxygen levels in a fish tank. Too little oxygen can distress the fish. They display more laboured breathing, they appear more lethargic and you’ll notice them spending more time near the surface of the water where there’s more oxygen.

If the situation is not resolved quickly, fish can eventually die.

Can A Fish Tank Have too Much Oxygen

Low oxygen levels in a fish tank is obviously bad and potentially fatal. What about too much oxygen? Is it just as dangerous?

First, we have to define what we mean by a fish tank being too oxygenated. What’s the right oxygen level and when does it become too much?

Different kinds of fish require varying levels of dissolved oxygen. Some species like betta and gourami can survive in low oxygen water thanks to their labyrinth organs. In desperate times, they can take oxygen directly from the air.

Some other species require oxygen rich water. But generally, a healthy aquarium should have between 80% and 110% of oxygen saturation or between 6 and 8 mg/L of dissolved oxygen.

Anything over 115% is considered over oxygenation, supersaturation or hyperoxygenation and it’s dangerous for the fish. In one case, after several fish at a trout farm in the Czech Republic died, researchers found that the water had an oxygen concentration of 136%.

Over-oxygenation Is a Stressor

Too much oxygen stresses fish, which can affect their appetite, movement and reproduction. Stressed fish are also more likely to suffer from infections and diseases.

In one study, researchers found that salmon raised in hyperoxygenated environments had higher levels of the stress hormone, cortisol. They were also more likely to suffer from various infections.

Too Much Oxygen Can Cause Lethal Gas Bubble Disease

Gas bubble disease is a potentially lethal condition that occurs when gas bubbles form in fish. This occurs when the water becomes too saturated with gasses, causing them to come out of solution and form bubbles.

High nitrogen concentration is the most common cause of gas bubble disease. But oxygen oversaturation can also cause it. If not quickly remedied, gas bubble disease can kill fish in a matter of hours or days.

Here’s a video that explains the dangers of gas bubble disease in fish.

Over Oxygenation Affects Water Quality

Too much oxygen indirectly affects other water qualities like the amount of algae, pH levels and the rate of plant growth.

Combined, all these effects can throw off the water balance, leading to problems with your fish.

What Causes Over-oxygenation?

Here are some possible causes of over-oxygenation in a fish tank. Note that excess oxygen concentration can occur all over the tank or in particular spots.

  • A waterfall feature can introduce excess bubbles in the water. Depending on the water temperature and salinity, this can lead to too much oxygen concentration particularly at the bottom of the tank.
  • Water changes can cause a sudden shift in water temperature. If you add water that is colder than the water in the fish tank, it can cause more oxygen to dissolve in the water and lead to oversaturation.
  • Too much algae or plants can add excessive oxygen in water during photosynthesis. This usually occurs if the fish tank is exposed to intensive lighting for long periods.
  • You can also go overboard with your oxygenation effort. If you have an air pump as well as an air stone and maybe an aerating filter as well, you can end up with oversaturated water.
  • A faulty or leaky pump can suck in air and introduce too much oxygen into the water. If you notice constant bubbling from the pump or tube, check whether it’s damaged.

How to Deal With and Prevent Too Much Oxygen in a Fish Tank

If you notice any behavioural change in the fish such as lethargic movement, increased or decreased appetite, the fish gasping or a tendency to stick to particular spots in the fish tank, immediately check the oxygen levels in the aquarium.

You can use a dissolved oxygen meter or test strips to measure oxygen level in the water. If it is above 115% or 8 mg/L, take emergency actions to fix the situation. What you do depends on what’s causing the oversaturation in the first place.

You can try removing the air pump or waterfall feature, remove some of the plants or do an emergency water change. Using a heater to raise water temperature to the right level can also help reduce oxygen concentration.

Constantly monitor oxygen levels to make sure they don’t go back to dangerous levels.

Here are some tips to prevent oversaturation from happening.

  • When doing water changes, go slow and make sure the new water is the same temperature as the water in the fish tank.
  • Be careful not to overplant your aquarium or let it become overrun with algae.
  • Limit how much sunlight or artificial light your aquarium is getting. For a new aquarium, 6-8 hours a day is enough. For an established aquarium, 10-12 hours a day is standard.
  • Use only one or two aeration equipment in the fish tank. You don’t need a dozen items bubbling away in the water.