What exactly is PHYTOPLANKTON?

What exactly is PHYTOPLANKTON?

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What exactly is PHYTOPLANKTON?

By scientific definition*, phytoplankton is a group of photosynthetic organisms adapted to living in water periodically or permanently. It includes both prokaryotic organisms (such as cyanobacteria) and eukaryotic organisms. Sometimes phytoplankton is referred to as “planktonic algae.”

In aquarium terms, phytoplankton is suspended microalgae in the water, serving as natural food for corals, fish, and filter feeders.


The importance of phytoplankton in a marine aquarium

While managing a marine aquarium, it’s important to be aware that phytoplankton is the beginning of the food chain, playing a crucial role in aquatic ecosystems. You’ve probably heard about the food chain in a biology lesson. However, assuming not everyone remembers what it involves, I’ll explain the concept.

The food chain represents the feeding relationships between different organisms in an ecosystem. In general, it describes the transfer of energy and matter from producers, which are photosynthetic organisms (e.g., phytoplankton), to consumers, which are organisms that feed on other organisms. The food chain illustrates the flow of energy and nutrients from one organism to another. A simplified diagram of the transfer of energy and matter is presented below.

food chain

Most aquarium enthusiasts use live phytoplankton due to its positive impact on aquarium ecosystems. It is undoubtedly one of the best natural foods we can provide for corals, invertebrates, filter feeders, and many other animals in our aquarium. Phytoplankton serves as both direct food for zooplankton and indirect food for corals, fish, and filter feeders.

The role of phytoplankton in marine aquariums


Phytoplankton contains photosynthetic organisms capable of photosynthesis. Importantly, they produce proteins, fats, and carbohydrates essential for the life of aquatic organisms. For some organisms, such as soft and hard corals, phytoplankton is their primary food source. Corals can extract nutrients from the water, including minerals and organic compounds provided by phytoplankton. Therefore, it is a crucial element in marine aquariums, supplying essential nutrients and supporting the healthy development of marine organisms in artificial environments. Phytoplankton contributes to the growth of corals and enhances their polyp extension.


Moreover, phytoplankton is responsible for producing a significant portion of atmospheric oxygen (O2) and absorbing carbon dioxide (CO2). It participates in the regulation of carbon dioxide and oxygen cycles (1):

   CO2 ⇆ O2    (1)

Similar to land plants, phytoplankton contains chlorophyll (a pigment) that gives the green color to phytoplankton. Additionally, chlorophyll can use light to transform carbon dioxide and water into organic compounds (such as glucose – C6 H12O6 ), serving as a food source for other organisms and producing oxygen (2):

6 CO2  + 6 H2O   ⇆  C6H12O6 + 6 O2 (2)


By using phytoplankton, we also counteract the outbreaks of cyanobacteria. Phytoplankton acts as a natural opponent to cyanobacteria due to food competition, which can lead to a reduction in the growth and development of cyanobacteria in the aquarium. Moreover, supplementing marine tanks with phytoplankton supports the formation of bacterial flora and microfauna.

Phytoplankton composition

The size and composition of phytoplankton depend on various factors, such as nutrient availability, water temperature, light intensity, and the presence of chemical components. Phytoplankton includes not only single-celled microalgae floating in the water but also bacteria (e.g., floating photosynthetic cyanobacteria) and coccolithophores – single-celled protists with a calcareous shell structure. The common feature of phytoplankton is that they are single-celled organisms, floating in the water column, conducting photosynthesis, and producing oxygen. The most well-known representatives of phytoplankton are:

  • Diatoms: one of the most numerous and diverse groups of phytoplankton. They are known for their beautiful and intricate geometric shapes
  • Cyanobacteria: often referred to as blue-green algae. They have the ability to fix nitrogen
  • Green Algae: photosynthetic green algae 

It’s important to remember that using phytoplankton of unknown origin can lead to water contamination with undesirable elements such as copper or other trace elements. Choose and use only high-quality and pure products!

Unlike most chemical preparations or supplements, phytoplankton is a living product and should be properly stored. This is crucial for its effectiveness and nutritional value. If it is not stored under appropriate conditions, degradation, death, and loss of its valuable properties for marine organisms can occur.

Problems related to the deficiency or excess of phytoplankton in seawater

Phytoplankton is a great natural food for marine aquariums. Unlike dry foods, it doesn’t cause issues in the tank and gets eaten up by the aquarium inhabitants.

In aquariums, there’s often not enough phytoplankton. This happens because advanced water cleaning systems in marine tanks remove it. The demand for phytoplankton is usually higher than what’s available in the tank.

But sometimes, there’s too much phytoplankton. This usually occurs when you add too much compared to the aquarium’s size and the number of animals living in it.

How to protect your aquarium?

Regularly monitor the amount of phytoplankton in a marine aquarium and maintain it at the right level. 

How to do it? There’s no better way than regular observations and testing of parameters (such as nitrates and phosphates) in the tank. You’ll surely notice if something worrisome is happening. 

Below are some tips on what to pay special attention to when using phytoplankton.

Indicators of abnormal phytoplankton levels in a marine aquarium


  • Disturbances in the process of exchanging CO2 for O2
  • Increased concentration of nitrates and phosphates
  • Algae growth
  • Increased susceptibility of corals to infections
  • Weakened skeletal growth
  • Color fading


  • Disturbances in the process of exchanging CO2 for O2
  • Tank turbidity
  • Weakened skeletal growth
  • Chemical balance disorders, including a decrease in the concentration of nitrates and phosphates


To maintain the right level of phytoplankton in your aquarium, you should dose it according to the manufacturer’s recommendations and ensure it stays at the proper level. If the recommended level is exceeded, lower the parameter value in the water. Conduct up to 6 water changes, replacing about 15% of the aquarium water each time, until you reach the recommended parameter value. The replacement water must have the correct target salinity. Use salt with the appropriate parameters and composition suitable for an ICP test.

If the phytoplankton level is low, consider using phytoplankton from a supplier known for high-quality and pure products to balance it. For a consistent phytoplankton level in a marine aquarium, we recommend systematically supplementing this component based on the coral population in your tank.


Regularly dosing phytoplankton in appropriate amounts into a marine aquarium supports coral nutrition, growth, and proper development, enhancing their vibrant colors. It’s essential to use high-quality phytoplankton from a trusted source and store it correctly.

* Wilk-Woźniak E. 2016. Fitoplankton. W : Sądag T., Banduła T., Materek E., Mazurkiewicz-Boroń G. & Słonka R. (red.) Zbiornik  wodny Dobrzyce-monografia, Kraków: RZGW, MPWiK: 158-166.

About author

Picture of Magdalena Metzler

Magdalena Metzler

Privately, I am a mother and a lover of nature and sport. My main interest is quantum chemistry, which hides a whole lot of unsolved mysteries and connections, which is extremely exciting from a scientific point of view.
In my scientific career, I have conducted international projects focused on innovative solutions for many branches of business, e.g. automotive, construction, and now, of course, marine aquaristics.
Working at Reef Factory gave me a passion for marine aquaristics, which I can develop every day, building a chemistry department and creating products that will help aquarists take care of tanks and ensure the highest safety of animals. One of the most exciting memories of working at Reef Factory is the commissioning of the ICP-OES spectrometer, which analyzes the elemental composition of seawater. The method of analysis in ICP is based on an analytical technique, which is a combination of my passion for quantum chemistry and marine aquaristics.
I hope you find my articles on ReefPedia interesting and helpful! Happy reading :))