Last Updated on February 10, 2023 by Matt

Phosphate has the chemical formula PO₄³⁻ and is found in every aquarium. Phosphate in aquarium tanks is essential as it provides a source of phosphorus vital for plant growth. 

However it is exactly this property, it’s use as a vital fertilizer, that can make it problematic if the concentration gets too high. 

In this article we will discuss why high phosphate levels aren’t good news, how phosphate is introduced into aquariums, the effect it has in tanks, and how to reduce and keep phosphate levels low. 


Why Are High Aquarium Phosphate Levels bad?

High concentrations of phosphate in aquarium tanks is bad for a number of reasons. It is an inorganic form of phosphorus, and is a key fertilizer and way that plants obtain the phosphorus they need. This is why it is often found in aquarium plant fertilizers.

Phosphorus, along with nitrogen, is a key element for life. It is a key component of DNA and RNA, and is used by plants for photosynthesis, respiration, and transfer and storage of energy.

Therefore a tank with live plants needs some phosphate for them to be healthy. However when the phosphate levels get too high, they can cause issues. High phosphate levels are a symptom of old tank syndrome, which often affects mature established tanks.

The problem is that phytoplankton, or algae, are essentially extremely small plants and use phosphate as a source of phosphorus too. When in small concentrations and everything is in sync, there isn’t enough phosphate for the algae to bloom. However when the phosphate concentration rises, algae can explode into a bloom. 

Algal blooms lead to a reduction in water oxygen, and will also raise the pH of the water. This will lead to ammonia and nitrite spikes, causing ammonia poisoning and nitrite poisoning. All of this can lead to severely unwell fish, and will cause deaths if not dealt with quickly. Algal blooms are also notoriously hard to completely remove from aquariums. 

Effect of High Levels of Phosphate in Aquarium Tanks

Effect of High Levels of Phosphate in Aquarium Tanks

High concentrations of phosphate will affect freshwater and saltwater tanks differently. But as an overarching impact, you will see the effect on the algae in the tank. 

In saltwater tanks green hair algae can become a particularly big problem. Algae will grow quite exponentially. Corals are also impacted by high phosphate levels, as they have a symbiotic relationship with photosynthetic zooxanthellae algae. In high phosphate water, brown algae can become an issue in the coral tissue, giving the coral a brown color and masking their usual bright coloration. This brown algal growth can also impact the corals ability to form its calcium carbonate skeleton by blocking the uptake of calcium carbonate from the water column

As there are less direct rivals to algal growth in saltwater tanks – you will rarely find a planted saltwater tank – these setups can handle a lot less phosphate until algal blooms become an issue. 

Freshwater tanks are regularly planted. Aquascaped tanks can use a great deal of phosphate and so can handle a higher concentration. 

How Does Phosphate Get Into Aquariums?

Phosphate is a natural compound and so can get into the water column in a number of ways. Phosphate will be released into the water column through the natural breakdown of waste in the tank. Excess food, fish feces, plant matter, all of these will release phosphate as they are decomposed. 

It can also be introduced from outside the tank as well. Tap water may well have phosphate in it, and fish food definitely will. Buffering chemicals in carbonate hardness and general hardness buffers can also contain a significant amount of phosphate. Filter media such as activated carbon may leak phosphate back into the water column.

Common sources of phosphate in aquarium tanks are:

  • Fish feces
  • Plant matter
  • Fish food
  • Tap water
  • Fertilizers
  • Decomposing algae
  • KH and GH buffers
  • pH buffers
  • Chemical filtration media

What Is a Healthy Level of Aquarium Phosphate?

The level of phosphate that is healthy in aquariums depends on whether you have a freshwater or saltwater setup. 

Freshwater tanks can handle a significantly higher concentration of phosphate compared to saltwater tanks before excess algal growth becomes a problem. 

In general, a freshwater tank should have between 0.1 – 1.0 ppm (mg/L) of phosphate. As a general rule as well, the concentration shouldn’t exceed 3.0 ppm or you are risking an algal bloom.

In freshwater tanks which are highly planted, such as aquascaped tanks, then higher phosphate levels may be necessary to give the plants the nutrients they need. They may need a concentration of 1.5 ppm for instance. Keep a careful eye on algal growth and the plant growth. You don’t want plants to brown because they haven’t got enough nutrients, but cranking up the nutrient level too much will fuel algal growth.

For marine saltwater tanks, the picture changes dramatically. The target is not to exceed a phosphate concentration of 0.05 ppm. A healthy saltwater reef tank should have a phosphate concentration lower than 0.02 ppm. As you can see that is significantly lower than in freshwater tank systems. Any higher than this and the health and growth of your corals will begin to be negatively impacted.

You should also take into account if you are looking to breed any of your tank inhabitants. For instance high aquarium phosphate levels have been thought to have a hand in failed shrimp breedings.

You should regularly be testing for phosphate, no matter if you have a freshwater or saltwater aquarium. A great phosphate testing kit is the Seachem Phosphate Test Kit. You can also get a full test kit like the API Master Test Kit which tests for phosphate along with multiple other indicators of water health. 

How To Reduce Aquarium Phosphate

How To Reduce Aquarium Phosphate

There are a good few things that you can do which will reduce levels of phosphate in aquarium tanks. If you have tested your water and found high phosphate levels then you should take the following steps in order:

Large scale water change

When it comes to removing compounds from the water column, sometimes the simplest option is the most effective.

That’s why when you have tested high levels of phosphate in your aquarium the first thing you should do is a large scale water change of 50%. This will instantly remove a lot of the phosphate floating around in your tank, and hopefully remove some free living algae as well. Water changes can be made easier using a product like the Python water changer.

Before you put the new water into your tank you should ensure that it is the same temperature. A sudden jump, even of a few degrees, up or down will stress your fish. 

Also ensure that you use a good water conditioner to make the tap water safe. Tap water contains compounds such as chlorine and heavy metals that are toxic to fish. Some tap water sources are also high in phosphate, so make sure to check you aren’t putting more phosphate back in! The best option for water changes is to use distilled or RODI water, as they won’t contain any phosphate at all.

Full cleaning

If you have conducted the large scale water change and you are still seeing higher than normal phosphate levels when you test the water, then it’s time to do a full cleaning of your tank. 

You should use a gravel vacuum to clean the substrate and remove any decomposing organic material. 

Also important is to properly clean the filter. Not cleaning the mechanical filtration media properly will result in decaying matter release phosphate into the water column. Replacing chemical filtration media regularly is important to ensure its effectiveness doesn’t diminish. 

You should try to remove as much algae as possible from your tank. Scrape any algae from the glass, but be careful not to scratch it, especially if you have a plastic tank. 

Phosphate removing filter media

There are many aquarium phosphate removers on the market. Using one in the chemical filter of your filtration system can help to reduce phosphate levels. 

You should put them in the filter after you have performed the water change and cleaned the tank. This should remove the last of the phosphate that remains. 

Keeping phosphate filter media in the filter will also help to keep phosphate levels low in the future.

How To Prevent Aquarium Phosphate Spikes

Preventing phosphate concentration spikes is the best way of dealing with the problem. The consequences of increased phosphate levels are algal blooms, and it can be extremely hard to rid your tank fully of algae once it takes hold. 

You can do the followings things to help prevent phosphate levels from increasing in the first place:

Regular Maintenance: It might sound obvious, but regular water changes and cleanings will help the most in keeping your phosphate levels to a minimum. A weekly water change of 15% will greatly help. As will using an aquarium vacuum cleaner to remove decomposing debris on the substrate. Algae can grow on all surfaces, including the glass and decorations; cleaning this off will help keep phosphate low. Cleaning your filter regularly will also reduce sources of phosphate.

Don’t Overfeed: Overfeeding your fish will not only have the impact of directly making them more unhealthy by making them overweight and possibly causing swim bladder diseases. Feeding your fish too much will also mean more waste, which then decomposes and releases phosphate into the water. It will also release ammonia as well. You should only feed your fish what they can finish in 2-3 minutes.

Filtration Media: Activated carbon is the most common chemical filter media but may leach phosphate into the water. Combining carbon with a phosphate adsorber can be a good idea, as you will get the benefits of both. A good option is Seachem PhosGuard.

Seachem PhosGuard, 100 mlL
  • Rapidly Removes Phosphate And Silicate From Marine And Freshwater Aquaria
  • Highly Porous For High Capacity And Bead Shaped For Optimum Water Flow
  • Outperforms all competing products
  • Not Recommended For Phosphate Based Buffers In Freshwater Aquaria
  • 100 milliliters treats 60 US gallons


High phosphate levels cause huge problems in reef tanks, where it interferes with corals’ ability to uptake calcium carbonate and therefore inhibits growth. It will also fuel algal growth. High phosphate levels are often followed by algal blooms, which are not only unsightly but also terrible for the health of your fish. 

Hopefully this article has given you the information on phosphate in aquarium tanks that you were looking for. You should now know what phosphate is, where aquarium phosphate comes from and how to reduce phosphate and keep levels low. 

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