Filtration Why Anaerobic Bacteria is Beneficial for Aquariums

Why Anaerobic Bacteria is Beneficial for Aquariums

Anaerobic bacteria removes nitrate from an aquarium by converting it into nitrogen gas, reducing the need for water changes. It also produces toxic hydrogen sulfide gas (H2S), which concerns many fishkeepers. However, H2S is safely converted into harmless sulfate in the presence of oxygen.

Anaerobic bacteria is a contentious topic in many fishkeeping circles because of a few misconceptions about its function. The truth is that anaerobic bacteria plays a tremendously useful and beneficial role in the aquarium hobby.

In this article we will explore why:

How does anaerobic bacteria benefit an aquarium?

Anaerobic bacteria operates similarly to the aerobic nitrifying bacteria that converts ammonia into nitrate. However, there are a few key differences.

Anaerobic bacteria removes nitrates in the process of denitrification

The main benefit of having anaerobic bacteria in an aquarium is that is removes nitrate and converts it into nitrogen gas.

Here’s how that works:

How aerobic bacteria acquire energy

Most creatures acquire energy from food. The food contains energy that is trapped within it. In order for the creature to receive this energy, it requires an oxidising agent.

The most effective oxidising agent is oxygen. What happens is the creature breathes in the oxygen and the oxygen converts the food source into another compound. During this oxidation process, energy is released from the food source and consumed by the creature.

Creatures that use oxygen as an oxidising agent are considered aerobic. Humans are aerobic creatures, as are nitrifying bacteria in our aquariums that convert ammonia into nitrate.

How anaerobic bacteria acquire energy

Anaerobic bacteria live in anoxic environments in an aquarium. These are places with little or no oxygen present, such as in special denitrifying filters or deep substrate beds.

Because there is no oxygen, anaerobic creatures have evolved to use oxidising agents other than oxygen. Examples of other oxidising agents include nitrogen and sulfate. They are not quite as effective as oxygen, but will do the job when no oxygen is present.

The main anaerobic bacteria we talk about in our aquariums are those that breathe in nitrogen and use it as an oxidising agent to extract energy from its food. Specifically, it uses nitirogen from nitrate (NO3) and uses it to extract energy from carbon-based food sources.

As part of this process, an aquarium’s nitrate levels will lower because the anaerobic bacteria is using it as an oxidising agent, which converts it into harmless nitrogen gas (N2).

It then bubbles to the top of the tank and escapes into the atmosphere.

This is beneficial because high concentrations of nitrate is toxic for fish.

Denitrification eliminates the need for water changes

The primary purpose for performing water changes is to lower the concentration of nitrate in an aquarium. Anaerobic bacteria removes the need to do this because it is removing nitrate itself by converting it into nitrogen gas.

Why do fishkeepers avoid creating anaerobic bacteria?

In addition to the beneficial anaerobic bacteria that removes nitrate, there are other sulfate-reducing bacteria that live in anoxic environments as well.

It produces hydrogen sulfide gas that is toxic to fish

This bacteria breathes in harmless sulfate and converts it into hydrogen sulfide, which is a gas that is very toxic to fish. This toxic gas is the primary concern that causes weary fishkeepers the avoid anaerobic bacteria at all costs.

Fishkeepers believe that by having anaerobic bacteria in their aquarium, they will kill their fish with toxic hyrdogen sulfide gas.

What most people don’t understand is that hydrogen sulfide gas is quickly and readily neutralized before it has an opportunity to come into contact with fish.

Here’s why:

Why hydrogen sulfide gas is safe for aquariums

It neuturalizes into sulfate in the presence of oxygen

Hydrogen sulfide gas converts back into harmless sulfate when it comes into contact with oxygen. So as the hydrogen sulfide gas bubbles up and starts rising from the anoxic bottom layer of a deep substrate tank, it passes through an aerobic layer of sustrate that contains oxygen.

This oxygen-rich layer safely converts the hydrogen sulfide into sulfate before it enters the water column where your fish are swimming.

However, even if this layer of substrate fails to convert the hydrogen sulfide, there are a few additional factors to consider:

It is easy to detect

Humans have evolved to easily detect the smell of hydrogen sulfide gas because it’s toxic to us as well. As a result, you will be able to quickly identify the rotten egg smell emitting from your aquarium when it’s present in even very small concentrations.

A person will be able to smell this gas before it is in high enough concentrations to kill your fish. Meaning, you will be able to respond before anythign bad happens.

It is easy to prevent

If you detect hydrogen sulfide it means there is not enough oxygen in the tank to convert it into sulfate. A simple and easy solution is to put an airstone in the tank to increase oxygen levels.

By increasing the oxygen levels you will effectively eliminate any hydrogen sulfide that may be present if your tank.

The thing is, the presence of hydrogen sulfide in the water column is evidence of an even larger problem that needs to be addressed anyways:

Its presence in the water column means there is little oxygen

Like it was mentioned above, the only way hyrdogen sulfide poses a risk to your fish is if there is no oxygen in the tank to convert it to sulfate. But if there is no oxygen in the tank then your fish will be unable to breathe themselves, which is a far greater problem.

Your fish will likely show signs of oxygen deprevation and die of suffocation before hydrogen sulfide can become a problem in the first place.

Ryan Ferguson

Founder, Rooted Tank

Ryan Ferguson, the founder of Rooted Tank, started fishkeeping in 2019. He has continued to level-up his planted aquarium skills and wanted to share his journey and knowledge with other aquarists.


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