Can You Add Too Much Bacteria to a Fish Tank?

Estimated read time 6 min read

Adding bacteria to a fish tank can be tricky. Over time, excessive amounts of ammonia build-up, causing an unhealthy environment for the fish. Beneficial bacteria consume this excess ammonia and help to restore balance in the tank.

But, how much should you add? Can you add too much bacteria to a fish tank?

Since beneficial bacteria consume ammonia and waste, adding too much will cause them to die off. While it is essential to follow the manufacturer’s directions, if you go over this amount slightly, time and the aquarium filter will eventually correct the issue and restore the tank to its perfect balance.

Do you want to learn more about how the bacteria in a fish tank work and the appropriate amounts?

Keep reading.


What Are Beneficial Bacteria?

In a natural aquatic environment, beneficial bacteria are naturally occurring and assist with the breakdown and processing of organic matter. These bacteria also help reduce the damage caused by harmful algae blooms that endanger fish and other marine life.

In an aquarium, beneficial bacteria are added to the water to help break down excess food and waste and keep the environment clean and healthy for the fish.

Types of Bacteria in Your Aquarium

There are hundreds of different types of bacteria inside an aquarium, and those levels are in a state of near-constant change or adjustment based on the organism’s actions living in the tank. These organisms include both fish and plant life.

Heterotrophic Bacteria

Heterotrophic bacteria are decomposers or scavengers. These bacteria consume dead plant and animal tissue to survive. Heterotrophic bacteria are divided into three main types;

parasitic, which only grow inside a living host cell; saprophytic, which break down complex organisms into more straightforward forms fish can use; and symbiotic, which hold a mutually beneficial relationship role in keeping ammonia levels in line.

Autotrophic Bacteria

Autotrophic bacteria differ from heterotrophic bacteria by producing food rather than consuming organic matter. Autotrophic bacteria include algae, which plays a vital role in the ecosystem of the fish tank by producing oxygen through photosynthesis. The bacteria make food using light, heat, or carbon dioxide.

Where Are the Beneficial Bacteria in an Aquarium?

Beneficial bacteria are typically located throughout the aquarium on the glass, on rocks in the gravel. But adding biological filter media will provide the bacteria with an ideal environment to flourish. An increased amount of beneficial bacteria will produce an efficient system for the disposal of aquarium waste and a healthier fish tank.

Why Are Beneficial Bacteria Important?

Beneficial bacteria play a big part in keeping a clean aquarium daily and include the two types of bacteria species necessary to complete the nitrogen cycle, which breaks down aquarium waste.

What Does Bacteria Do?

Bacteria play many roles inside the aquarium ecosystem, including breaking down waste products and releasing oxygen and other beneficial gas back into the water.

What Happens If You Don’t Have Enough Beneficial Bacteria?

Too little beneficial bacteria is a common issue with a new aquarium setup when there are insufficient bacteria to break down the organic waste in the tank, harmful ammonia, and nitrite build-up, causing an unhealthy environment for the fish.

Why Is Ammonia So Deadly?

When ammonia levels in the tank are too high, it sticks to the bodies of the fish. In addition, ammonia causes stress and damage to their gills when attached to their bodies.

Signs of Ammonia Poisoning

Signs of ammonia poisoning include a red or lilac tinge on a fish’s gills, lack of appetite, and lethargy. The fish may also appear to be gasping for air at the surface.

How Much Ammonia Is Too Much?

Any amount of ammonia above 0.0 ppm is considered dangerous and could make the fish ill. Only deep cleaning ⅓ of the gravel of your tank in any given week will help keep the majority of the bacteria in the tank.

Fixing Ammonia With Good Bacteria

Adding some stone gravel and live plants to the aquarium will help reduce the amount of ammonia inside the tank. The stone provides a place for the bacteria to set up colonies quickly, and the plants will use the excess ammonia for fertilizer.

Lastly, it’s essential to know how to lower ammonia in a fish tank.

The Nitrogen Cycle

Understanding how nitrogen is crucial to all life on the planet will explain the vital role of this element in the aquarium ecosystem. The nitrogen cycle consists of three steps: nitrogen fixation, nitrification, and denitrification. The cycle is controlled by bacteria and converts the nitrogen into a form that the ecosystem can use.

How to Add Bacteria to an Aquarium

Adding bacteria is as simple as using a product like Seachem Stability. Just follow the instructions and dose based on the size of your aquarium.

You can also encourage the growth of bacteria by turning off the aquarium light, and increasing the temperature of the water will promote growth for beneficial bacteria. Adding gravel and letting the filter run will increase the tank’s oxygen level, which should help with healthy bacteria growth.

Related: Does API Quick Start work?

How to Build Up Good Bacteria in a Fish Tank

Regularly testing your tank’s water to ensure the correct ph will help build up good bacteria.

Excellent Sources of Beneficial Bacteria

The following are excellent sources for quickly building good bacteria levels inside the tank.

  • Filter media from another established tank (Best)
  • Soil (Use with caution)
  • Compost (Use with caution)
  • Bottled bacteria (Easiest Method)

Does Adding Water Increase Good Bacteria?

Bacteria can reproduce faster in a warm environment, so warm water and limiting UV exposure will promote healthy bacterial colonies.

How Long Does It Take for Beneficial Bacteria to Grow in an Aquarium?

A new tank will take four to six weeks to complete the nitrogen cycle for the beneficial bacteria.

How Often Should You Add Bacteria to an Aquarium to Maintain Healthy Levels?

You should add bacteria to the water each time you clean the tank, add new fish or have a sick fish.

Frequently Asked Questions

Here are a few more quick facts about fish tank maintenance.

Can you add bacteria to a fish tank with fish in it?

Yes, you can add bacteria to a fish tank with fish in it.

Maintaining an aquarium full of fish will require monitoring and adjustment to the bacterial levels of your tank.

Can you add too much nitrifying bacteria?

Adding too much nitrifying bacteria to an aquarium is almost impossible, so don’t worry if you add a bit more than recommended.

Learn more about how to use additives in our full article on some of the best nitrifying bacteria products that you could use in your aquarium.

How much bacteria do you need to add to your fish tank?

If you are adding bottled bacteria, it’s best to follow the instructions but the natural bacteria that develop are self-regulating.


Regulating the number of healthy bacteria in your aquarium can seem intimidating, but maintaining it healthy is more straightforward once you’ve established the tank.

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