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Can You Add Too Much Bacteria To A Fish Tank?

can you add too much bacteria to a fish tank

When you hear about bacteria in a fish tank, is it bad news? Bacteria has gotten a bad rap, as most are harmless, neither good nor bad; however, harmful bacteria can kill your fish, which is why adding beneficial bacteria is essential. But is it possible to overdo it and add too much good bacteria to a fish tank?

Well, in this article we discuss if can you add too much bacteria in a fish tank or not, also you will know different types of beneficial bacteria. Similarly, you will also know why ammonia is deadly and how to add and maintain the necessary bacteria for a healthy ecosystem in your tank.

Can You Add Too Much Bacteria To A Fish Tank?

Adding more beneficial bacteria will not harm your fish or plants. In fact, beneficial bacteria convert ammonia and nitrites into nitrates, which help to improve the health of your tank. However, it is important to note that too much bacteria in a fish tank can lead to a bacterial bloom.

Bacteria can grow rapidly in water, causing the water to become cloudy. Bacteria blooms are usually harmless, but they can be unsightly.

If you’re worried about adding too much beneficial bacteria to your tank, you can start monitoring water quality by adding small quantity first. If the water quality stay clear, you can add more bacteria if needed

What Are Beneficial Bacteria?

Most of the 5 nonillions or more bacteria on earth (a nonillion has 30 zeros) are beneficial. Without bacteria, we would not be able to digest food, have antibiotics, or enjoy fermented foods. Some beneficial bacteria keep us healthy by attacking harmful bacteria, while others prevent diseases by occupying the spaces that harmful bacteria desire.

These single-celled organisms are so plentiful that it is estimated a gram of soil can host 40 million bacteria cells.

Bacteria can be grouped in different ways:

  • For example, there are three main shapes—spherical, rod-shaped, and spiral.
  • Where they live is another way of grouping them. For example, aerobic bacteria need oxygen to grow, while anaerobic bacteria cannot grow where oxygen is present. Facultative anaerobes are not as picky—they can live without oxygen but prefer environments where they can access oxygen. They live in soil, vegetation, and water.
  • Another way of grouping bacteria—by how they feed—is essential to understand how bacteria can be beneficial in a tank.

Why Are Beneficial Bacteria Important?

Understanding how bacteria feed helps in understanding the relationship between bacteria and conditions in your tank. Photoautotrophic bacteria get their energy through photosynthesis. Although some create oxygen, they do not produce it in high enough quantities to make much of a difference in a fish tank.

Aquarists pay more attention to two other bacteria types: heterotrophic and autotrophic. Since each plays an important role, both are needed in a healthy tank. Problems occur when there is an incorrect balance between the two.

Heterotrophic Bacteria Are Dependent on Organic Material

Heterotrophic bacteria consume organic material—dead plant matter, fish waste, and uneaten food. The organic material they eat is mineralized into compounds such as ammonia. The slimy gunk that grows on tank walls and ornaments is probably filled with heterotrophs. Also, the bacteria you see in bacterial blooms are typically heterotrophs.

Heterotrophic bacteria generally do not hurt fish directly, but they can cause a serious problem when left unchecked. First, they turn aerobic and suck up the oxygen in the water. This causes fish to head to the water’s surface in search of air.

Second, as they digest the organic carbon, heterotrophic bacteria release ammonia, and ammonia, as we all know, is deadly in a tank. So, how are bacteria involved in eliminating ammonia from tank water?

Heterotrophs are larger than most autotrophs and reproduce more quickly. While it might take an autotroph 24 hours to reproduce, a heterotroph can do so in 20 minutes. This is one reason it takes ten to fourteen days to cycle a tank.

Autotrophic Bacteria Can Create Their Own Food

Instead of needing organic material to feed on, many autotrophic bacteria use chemicals for their food. Depending on where they live, these organisms will feed on iron, sulfur, hydrogen sulfide, and so on. In a tank’s ecosystem, autotrophs feed on ammonia.

These bacteria are sometimes called nitrifying bacteria. That name refers to what happens to the ammonia after the bacteria eat it.

  1. First, the bacteria break down deadly ammonia into less toxic nitrites.
  2. However, nitrites are also toxic, so additional nitrifying bacteria convert the nitrites into nitrates.
  3. Nitrates are even less toxic, and plants and algae feed on them.

Nitrifying bacteria play an essential role in keeping the ecosystem healthy for fish.

Why Ammonia Is So Deadly?

If we catch the flu, it can be deadly, but it can cause us to be sick for a time and then recover. Other things, such as rat poison, will most likely be deadly unless we get immediate help. Rat poison, therefore, is directly toxic.

Ammonia is directly toxic to fish, but what does ammonia do to a fish?

  • It burns fish tissue chemically, starting with the gills.
  • Once absorbed in the body, ammonia damages other tissues.
  • The damaged gills make respiration more difficult.
  • Ammonia build-up occurs as the fish cannot effectively excrete the ammonia.
  • When additional ammonia build-up occurs, it further increases the amount of ammonia in the fish’s system and compounds the damage.

The poisoned fish eventually dies from the lack of oxygen and organ failure.

Signs of Ammonia Poisoning

The signs of ammonia poisoning are related to what the ammonia is doing to the fish.

  • Visible damage to the gills, such as redness and swelling, indicates the ammonia is “burning” the tissue.
  • Swimming to the surface to gasp for air is a sign the fish needs more oxygen.
  • Lethargic fish are spending most of their energy fighting off the effects of ammonia.
  • The jerky, darting movements are a sign the fish’s nerves and organs are being affected.

If you have an aquarium with coral or invertebrates, ammonia will affect them as well.

How Much Ammonia Is Too Much?

Instead of waiting for fish to show signs of ammonia poisoning, you can use a test kit to check the water for ammonia levels. In a perfect world, there would be no ammonia in the water, but levels above 0.06 need immediate attention.

With most test kits, each parameter, such as pH, chlorine, and ammonia, is tested separately, so you can purchase strips that test only ammonia, such as Tetra EasyStrips Ammonia Test Strips. You could also get a kit, such as the Alert Series Ammonia Alert. It continuously monitors ammonia, using a sensor that changes colors to signal how much ammonia is in the water.

Fixing Ammonia With Good Bacteria

Fixing water problems with bacteria is only part of the solution, but let’s examine the bacteria solutions first.

Several species of nitrifying bacteria are required to break down ammonia. The first is Nitrosomonas, which converts ammonia into nitrite. Nitrosomonas are an essential part of the nitrogen cycle and are often used in wastewater treatment. Since nitrite is still moderately poisonous, another species is required.

That species is Nitrobacter. These bacteria convert nitrites into nitrates. Scientists are not yet clear why two separate bacteria species are required to convert ammonia into nitrates.

The Nitrogen Cycle

can you add too much bacteria to a fish tank

To sum up, the nitrogen cycle works like this:

  1. Fish waste and expiration add carbon dioxide and ammonia to the water.
  2. The carbon dioxide is used by the plants or evaporates into the atmosphere.
  3. Nitrosomonas bacteria break down ammonia, releasing nitrites.
  4. Nitrobacter bacteria break down the nitrites and release nitrates.
  5. The nitrates are removed by the plants and water changes.

In nature, this process is an endless cycle that requires little intervention from humans. However, a fish tank is a closed system, and you need to create a system where beneficial bacteria can thrive.

Even then, the system needs additional help in the form of partial water changes and making sure the tank’s ecosystem has adequate beneficial bacteria. So, how does one add beneficial bacteria to a tank?

Does Adding Water Increase Good Bacteria?

Some people mistake adding freshwater with increasing beneficial bacteria. Replacing water reduces ammonia levels, but it does not add a lot of beneficial bacteria. That’s because only a limited amount of bacteria live in the water column.

Instead, healthy bacteria live on surfaces in the water—gravel, plants, filter, sponges, and so forth. So, while adding water from a healthy tank will provide your tank with cleaner water, it isn’t going to give the tank better bacteria.

If you are a newbie, the water column refers to all the water in a body of water from the surface to the floor. Even though it is called a column, picture layers, not columns. A fish that likes to swim on the bottom of the tank swims in the bottom column.

There are, however, ways to seed your water using inoculates or mediums that already contain good bacteria.

Excellent Sources of Beneficial Bacteria

Getting ammonia-eating bacteria in your tank can be done in numerous ways. Here are some excellent sources of beneficial bacteria:

Filter Media From an Established Tank

Aquarium filter material from an established tank is an excellent source of beneficial bacteria. The filter material from a tank that has been in use for two months or longer will have adequate amounts of bacteria living in it:

  • Sponges: When you squeeze a sponge filter, you should get some brown water. That will be an excellent source of good bacteria.
  • Gravel: Stir up some gravel if you use an under-gravel filter. The brown water will have plenty of beneficial bacteria. Bacteria need a high flow rate to flourish, so if you don’t use an under-gravel filter, the brown water you stir up won’t help much.
  • Other sources: Unwashed filter bags, ceramic rings, bio balls, and floss are also good sources of filter material.

Although some people say that used filters harbor diseases and contaminate new tanks, this is not usually the case. The pathogens you are likely to bring into the tank are ubiquitous, meaning they are in most tanks and on most fish.

An exception is filters from a tank with an outbreak of ich.


Soil houses the nitrifying bacteria that is required in a tank. You have two sources of soil:

  • Aquarium plants: Purchase several potted aquarium plants, making sure they come from tanks that have no fish or in containers. When you place them in the tank, squeeze the pots several times to release some of the bacteria in the plant’s roots.
  • Garden soil: If you have some dark garden soil, add a heaping tablespoon to the tank. The soil will have plenty of nitrifying bacteria. It also contains other tiny, helpful organisms that will do things like break up excess food. This action makes it easier for the good bacteria to do their job.

Avoid using commercial soil mixes as they contain little if any beneficial bacteria, perhaps because of the sterilization process they go through.


Compost will make an excellent inoculate. If you make your own compost, only use some if it has a similar consistency to the composted cow manure found at lawn and gardening stores. Add a tablespoon or two to the water.

Bottled Bacteria

Although the inoculates mentioned above make excellent inoculates, if you are tempted to buy some bottled bacteria, here are a few things to keep in mind:

  • Most bottled bacteria will not rush the initial cycling of your fish tank. You will still need to wait 10-14 days before adding water. The bacteria in the starter bottles are the same type of nitrifying bacteria found in other inoculates.
  • Bottled bacteria will introduce ammonia into a tank, so do not be alarmed if the label lists it. After all, the beneficial bacteria need something to feed on.
  • Bottled bacteria can’t hurt. Suppose you are nervous that your tank won’t get enough good bacteria, or you don’t have access to the inoculates mentioned earlier. In that case, some bacteria in a bottle will guarantee that you will have beneficial bacteria. A product such as API Quick Start works for both freshwater and saltwater aquariums.

How to Build Up Good Bacteria in a Fish Tank?

After introducing an inoculate into your tank, you can use several hacks to speed up the nitrogen cycle:

  • Up the oxygen: The nitrifying bacteria rely on oxygen to grow and thrive. Increase the water flow or add an air pump.
  • Up the temperature: Bacteria will reproduce more quickly in warmer water, so bring the water temperature up to the mid-80s °F (26.7 °C) when you cycle the tank. Do this before you add fish, as water this warm is dangerous to many fish.
  • Dim the lights: Nitrifying bacteria rely on ammonia as their food source, not light. Algae, however, use photosynthesis and carbon dioxide as their food. Keep your tank away from light sources while you cycle it.
  • Run the filter: It’s not only the fish and plants that need oxygen. Run the filter to provide oxygen to your nitrifying bacteria.

Bottled Bacteria for Maintenance Levels

If you are doing the things that you should do to keep your fish healthy and are worried that not enough ammonia is being converted to nitrates, there would be nothing wrong with adding some bacteria. As we said earlier, excess nitrifying bacteria will either go dormant or die if there’s not enough food for them.

However, keeping a tank healthy is tricky, and sometimes we do not get everything right. Take cleaning gravel. Not cleaning it often enough leads to excess waste but cleaning it too much can remove the helpful bacteria.

Other water conditions can also affect how well beneficial bacteria function. Low pH levels make it difficult for them to work correctly.

So, along with other maintenance, especially water changes, supplementing your tank with additional nitrifying bacteria can be helpful.

The Importance of Testing Your Water

Even if you are switching out your water according to schedule, keeping your tank clean, and making sure you don’t feed your fish too much, excess ammonia might not be stressing your fish.

  • If your tank receives too much direct sunlight and starts turning green, the phosphate levels in your water could be too high.
  • High iron content can lead to yellow-tinted water.
  • Other signs of illness in your fish can be caused by excess phosphates, incorrect pH levels, water hardness, or chlorine.

It’s important to test your water for more than just ammonia and nitrates. Testing strips such as API Test Strips are readily available.

How to Prevent Problems With Your Tank?

can you add too much bacteria to a fish tank
The best way to prevent problems with your tank is to use proper prevention techniques. This starts with setting up a tank so that ammonia can be turned into nitrates and then removing those nitrates with plants and regular water changes. You can also prevent some problems with losing beneficial bacteria by giving them plenty of surface area to live.

Those surface areas include gravel, plants, rocks, and filters. The type of gravel and rocks you use depends on the type of tank you have, but keep in mind that some rocks and gravel can change the tank’s water’s chemical nature. For example, if a rock releases calcium, the calcium will raise the water’s pH level.

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Bottom Line

The deal way to keep beneficial bacteria in your tank is to provide them good living space. Since bacteria live on surface areas, the more surface you give them, the more living space the good bacteria have.

It’s almost impossible to add too much good bacteria into a fish tank. Problems in a fish tank are usually caused by not having enough nitrifying bacteria to convert ammonia into nitrates or failing to switch out water to get rid of the less dangerous nitrates that your plants can’t eliminate.