Let’s find out how phosphorus affects fish in an aquarium.
This is another article about phosphorus. This time, we’ll focus on how phosphorus affects fish in an aquarium. If you haven’t read the previous articles Phosphorus in the aquarium – its role, forms and measurment and Phosphorus transformations in water yet, be sure to check them out. They provide a complete picture of this fascinating element and help you better understand how an aquarium works.
The role of phosphorus in fish
Fish can absorb some minerals directly from the water, but phosphorus is different. With its low concentration in water and limited uptake from it, fish obtain phosphorus mainly from their food. Phosphorus is an essential nutrient in a fish’s diet, boasting more known metabolic functions in vertebrates than any other element. Thus, any deficiency shows up as impaired body functions. An insufficient amount of phosphorus in the diet indirectly leads to metabolic disorders, resulting in slower growth and poorer feed utilization. Various skeletal deformities may also appear due to reduced mineralization of hard tissues.
Almost every aquarist has seen fish with a deformed spine, often blamed on genetic problems. The same issue is well known to food-fish farmers. For many years, spinal deformities plagued Atlantic salmon, until scientific research revealed they stemmed not from genetic disorders but from phosphorus deficiency during the early developmental stages of fry.
Fish phosphorus requirements
According to the literature, fish require between 0.3% and 1.5% phosphorus in their diet. As you can see, this is quite a wide range, mainly due to differences between species.
How is phosphorus absorbed from food?
All animal- and plant-based ingredients used in fish feeds contain phosphorus. The extent to which fish can utilize it, and how much is excreted into the water, depends both on the phosphorus content of the diet and on its bioavailability, which is determined by its chemical form and source. For example, fish meal used in feed production contains on average about 3.1% phosphorus, mainly in the form of calcium phosphate, which is poorly absorbed and largely excreted.
It is estimated that fish retain only about 20% of the phosphorus in their food, while 80% is released into the water. For this reason, modern feed formulation aims to minimize the amount of phosphorus entering the aquatic environment. One way to do this is by replacing fish meal with alternative protein sources that contain less phosphorus, often of plant origin. However, plants contain a large amount of phosphorus in the form of phytates, which fish also cannot digest efficiently. This means that in this case too, much of the phosphorus is excreted, leading to a decline in water quality.
Phosphorus absorption is also influenced by other dietary components. A fat-rich diet can increase phosphorus retention, while calcium deficiency can limit its uptake. Feed production technology also plays a crucial role. A major breakthrough in fish nutrition was the introduction of extrusion technology, which produces highly water-stable and easily digestible feeds. This reduces nutrient leaching into the water and decreases waste output, helping to maintain good water quality both in aquaculture and in home aquariums.
Feeding fish while protecting the environment
Because phosphorus released by fish contributes to water eutrophication, this topic has been widely studied. In particular, the nutritional needs of species such as trout and other salmonids raised in flow-through systems have been intensively analyzed. The goal was not only to determine how much phosphorus fish need, but also how much of it is actually retained versus excreted.
Two main strategies are used to reduce phosphorus waste: lowering its level in the diet or increasing its bioavailability. The latter involves feed additives that improve phosphorus uptake from poorly available sources. One of the most important of these additives is phytase, an enzyme that allows fish to digest phytates, the main form of phosphorus in plant ingredients (which can account for 50–80% of total phosphorus). Phytates are poorly absorbed and also bind calcium, magnesium, iron, and zinc, reducing their availability. Phytase breaks down these compounds, releasing usable phosphorus and improving the nutritional value of the feed. Organic acids such as citric acid can also increase phosphorus utilization. Even a 1% inclusion in feed has been shown to significantly improve phosphorus absorption, for example in rainbow trout.
Using phytase and organic acids allows fish to make better use of the naturally occurring phosphorus in feed ingredients, reducing the need for added inorganic phosphorus.
Moreover, mixing different protein sources in feed formulations improves phosphorus balance. Tropical, for example, uses diversified and balanced protein sources when formulating its fish foods, supporting both fish health and aquarium water quality.
Probiotic fish food and phosphorus absorption
Supplementing fish food with probiotics improves nutrient absorption, including phosphorus retention. This results from their positive effects on digestive processes, gut health, intestinal lining structure, and enzyme activity. Ultimately, probiotics improve digestion and nutrient utilization, including phosphorus. You can learn more in the article How do probiotics in fish food work?
The effect of dietary phosphorus on fish immunity
Limited data suggest that dietary phosphorus plays an important role in immune system function. For example, phosphorus deficiency was shown to reduce resistance in channel catfish (Ictalurus punctatus) to infections caused by Edwardsiella ictaluri. A dietary phosphorus level of 0.4% increased resistance, while 0.5% increased antibody production, indicating an even stronger immune response.
How do phosphates in water affect fish?
Phosphates in water are safe for fish. Toxicity tests (on fish, daphnia, and algae) have determined the concentration at which 50% of organisms die (LC50). For all three groups, this value was above 100 mg/L, meaning that only extremely high phosphate levels could be dangerous. In natural waters, phosphorus concentrations are usually only a few mg/L. Therefore, phosphates are not directly toxic. The real problem is their role in eutrophication. When large amounts of phosphates enter lakes or rivers, they stimulate excessive algae growth. When algae bloom and then die, oxygen is depleted, leading to poor water quality and low oxygen levels, which directly affects fish and other aquatic organisms.
Although few studies exist on the direct effects of waterborne phosphates on fish, I found an interesting scientific paper examining their impact on the welfare of juvenile African sharptooth catfish (Clarias gariepinus). The study aimed to determine a safe dose of phosphate fertilizer added to a recirculating aquaponics system, where regular water changes caused phosphorus deficiencies for plants. Three phosphate levels were tested: 40 mg/L, 80 mg/L, and 120 mg/L. The results showed no increase in fish mortality, and the best growth occurred at 40 and 80 mg/L. In the control group, with very low phosphate levels (<2.6 mg/L), phosphorus was mobilized from bones, negatively affecting growth. Importantly, no increase in cortisol (the stress hormone) was observed in any group.
However, the highest phosphate dose (120 mg/L) had a clearly negative effect on fish behavior and welfare. Fish more frequently surfaced to gulp air, indicating deteriorating oxygen conditions, and aggressive behavior increased.
In conclusion
I realize that the topic of phosphorus in aquariums is far from fully explored. Still, I hope that despite its complexity and the need to refer to basic chemistry, I have managed to present it in an accessible way. And if you still feel confused or overwhelmed, don’t worry. That’s completely normal. Phosphorus is just like that.
Literature
- Akpoilih B.U., Ajani E.K., Omitoyin B.O. (2016), Optimum Dietary Ca/P Ratio and Phytase for Growth and Bone Mineralization in Juvenile Clarias gariepinus Fed Soya Bean-Based Diet, J Aquac Res Development 2016, 7:2, http://dx.doi.org/10.4172/2155-9546.1000403.
- Debnath D., Sahu N.P., Pal A.K., Baruah K., Yengkokpam S., Mukherjee S.C. (2005), Present Scenario and Future Prospects of Phytase in Aquafeed – Review, Asian Australasian Journal of Animal Sciences 18(12):1800-1812.
- Eya J.C., Lovell R.T. (1997), Avaiable phosphorus requirements of food-size channel catfish (Ictalurus punctatus) fed practical diets in ponds, Aquaculture 154:283-291.
- Eya J.C., Lovell R.T. (1997), Available phosphorus requirements of food-size channel catfish (Ictalurus punctatus) fed practical diets in ponds, Aquaculture 154, 3-4:283-291.
- Hernández A.J., Satoh S., Kiron V. (2013), The effect of citric acid supplementation on growth performance, phosphorus absorption and retention in rainbow trout (Oncorhynchus mykiss) fed a low-fishmeal diet, Cien. Inv. Agr. 40(2):397-406.
- Lazzari R., Baldisserotto B. (2008), Nitrogen and phosphorus waste in fish farming, Boletim do Instituto de Pesca Sao Paulo 34(4):561-600.
- Nathanailides, C. i in. (2023), Addressing Phosphorus Waste in Open Flow Freshwater Fish Farms: Challenges and Solutions. Fishes 8:442.
- National Research Council (2011), Nutrient Requirements of Fish and Shrimp. Washington, DC: The National Academies Press.
- Strauch S.M. i in. (2019), Effects of ortho-phosphate on growth performace, welfare and product quality of juvenile African catfish (Clarias gariepinus), Fishes 4(1):3.
She studied environmental protection (specializing in surface water protection) at the Agricultural University of Wrocław (now the University of Life Sciences), where she later obtained her PhD at the Faculty of Biology and Animal Husbandry.
Her research focuses on the effects of diet, supplementation, and natural additives on the health, condition, and reproduction of aquarium fish.
In her free time, she cares for her fish and cats, tends to plants, enjoys hiking, and stays active through sports.
- Let’s find out how phosphorus affects fish in an aquarium. - March 9, 2026
- Phosphorus transformations in water? - February 23, 2026
- Phosphorus in the aquarium – its role, forms, and measurement - December 4, 2025
