Phosphates and their importance in marine aquariums

Table of Contents

What is phosphorus and what is phosphate?

Phosphorus and phosphates are two different but related topics in marine water chemistry.

Phosphorus (P, Latin: phosphorus) is a chemical element belonging to the XV group of periodic elements and is classified as a nonmetal. It generally exists in four forms: white phosphorus, red phosphorus, violet phosphorus, and black phosphorus. Each of these forms has unique physical and chemical properties.

In nature, phosphorus (P) does not occur in a free state due to its high reactivity. Phosphorus is present in many minerals such as apatite and phosphorites.

In turn, it occurs in seawater in various forms:

  • Inorganic Compounds (Orthophosphates) – the simplest and most common form of phosphates in the aquatic environment.
  • Inorganic Compounds (Polyphosphates) – long chains of phosphates linked together.
  • Organic Compounds (Organophosphates) – phosphorus compounds bound organically, meaning they are linked to carbon atoms.

The mentioned forms of phosphorus have different properties, effects, and impacts on life in a saltwater aquarium in various ways.

Where do phosphates come from in a slatwater aquarium?

  • Leftover fish food – decomposing releases phosphates;
  • Tap water – when using tap water instead of RO/DI water;
  • Low-quality salt may be contaminated with phosphates;
  • Dead organic matter, especially dead marine animals;
  • A malfunctioning protein skimmer may not remove enough organic compounds, leading to phosphate accumulation;
  • Excessive animal waste;
  • Rocks, especially ceramic ones, may contain phosphates;
  • Sand can be a source of phosphates;
  • Use of products that release phosphates as a side effect.

The importance of phosphates in a saltwater aquarium

Phosphate salts such as sodium phosphate (V) – Na3(PO4), which are commonly found in the natural environment, including seawater. Chemically, in water, phosphate salts break down into ions through a process called dissociation, and the form of phosphates depends closely on the water’s pH:

  • In water with a pH < 6, the dominant ions are H2PO4
  • In water with a pH between 6 and 8, the predominant ions are HPO42-
  • In water with a pH < 8.5, the dominant ions are PO43-

The pH value in a saltwater aquarium determines the form of phosphates in the water. This is another reason to maintain the proper pH level in a saltwater aquarium. High pH values (>8) favor the dominance of phosphate ions (PO43-), which are the most stable form and play a significant role in the marine ecosystem. Phosphates belong to the group of essential parameters in marine aquariums.

The appropriate level of phosphates in a saltwater aquarium is important for several reasons.

  1. Phosphates play a nutritional role and are one of the most important nutrients for many marine animals and algae.
  2. Having phosphates at the right level ensures a balanced phosphate cycle, which is a key element in maintaining a healthy marine ecosystem.

How to measure phosphates correctly?

Measuring the content of phosphates (PO43-) in a saltwater aquarium is crucial from an aquarist’s perspective. Phosphates have a significant impact on the aquarium environment, especially concerning algae growth and coral health. Although phosphorus is also measured using ICP-OES technique, phosphates are the most important in the context of saltwater aquariums. Orthophosphate PO43- is easily absorbed by algae and actively inhibits calcification when in excess. It’s important to control its level in saltwater.

There are many methods and products available on the market that allow for accurate determination of phosphate content in saltwater. Each product comes with instructions, and if followed correctly, will enable you to accurately measure this parameter.

The most important products enabling the determination of this parameter include:

  1. Home phosphate test kits – these are easy-to-use tests, such as the PO4 Smart Test Kit from Reef Factory. They allow for quick and relatively accurate determination of phosphate levels in the water.
  2. Photometric devices – more advanced devices, such as photometers from brands like Hanna, provide more accurate measurements.
  3. Automatic devices – advanced devices like the Smart tester from Reef Factory, which is integrated with the Smart Reef app, offer automatic monitoring of phosphate levels, allowing for continuous control and easier water parameter adjustment.

Tip: If algae blooms are noticeable in the aquarium and phosphate measurements indicate lower values than expected, it may be that phosphates have been consumed by algae, resulting in lower levels in the water. However, this does not necessarily mean that you do not have a problem with an excess of this parameter.

Correct phosphate content in a saltwater aquarium

Phosphates are one of the most important nutrient components for many marine animals and algae. Maintaining their levels is crucial, as some corals are sensitive to changes in phosphate levels. The level of this parameter should be adjusted based on the animals present in the aquarium. Lower values are recommended for SPS corals, while higher values are suitable for soft corals.

In simpler terms, this parameter should be set within the range of 0.06 to 0.15 ppm, with the optimal value being 0.08 ppm.

However, it’s important to stick to the recommended lower range (0.06 ppm) if SPS corals dominate the tank, and to the upper range (0.15 ppm) if soft corals are predominant.

Threats related to phosphate deficiency in seawater

Too low a phosphate content may cause:

  • the corals will start starving;
  • their metabolism will slow down;
  • some soft corals may begin to disappear;
  • coral tissue may also become thinner.

How to respond to phosphate deficiency?

In case the phosphate level is below 0.06 ppm, we recommend using products containing this nutrient to balance its level. To maintain a stable phosphate level in a saltwater aquarium, we recommend systematically supplementing this nutrient depending on the coral population in your aquarium.

Dangers related to excess phosphates in seawater

Too high a phosphate content may cause:

  • there will be a rapid growth of algae;
  • the corals will start to turn brown;
  • sensitive corals may die;
  • the calcification process will be inhibited.

How to reduce phosphates in a marine aquarium?

If the phosphate level exceeds 0.15 ppm, it should be considered as exceeding the recommended level. Please note that for sensitive corals (SPS group) this limit may be lower.

Find and eliminate the cause of the problem and reduce the value of this parameter in the water. Make up to 6 water changes. It is recommended to replace approximately 15% of the aquarium water volume during each replacement until the recommended value of this parameter is achieved. The water prepared for replacement must have the appropriate target salinity level. Use salt with the appropriate parameters and composition appropriate to perform the ICP test. You can find more about methods of lowering phosphates in a dedicated article.


Remember, maintaining the right level of phosphates is crucial for the health and balance of a saltwater aquarium ecosystem. Regular monitoring and appropriate corrective actions are necessary to maintain optimal conditions for marine life.

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 :))