The Redfield Ratio - Balancing Nutrients in a Saltwater Aquarium

Either you’ve clicked on this article because you have recently heard the phrase ‘unbalanced nutrients’, or someone has told you that your tank may be unbalanced with no other explanation. You may have had someone tell you that you should look into the Redfield Ratio. You may have Googled the phrase and found the image below, but not understood what you are looking at. So what exactly is the Redfield Ratio?

what is the redfield ratio

The table above is used to determine your likelihood of getting algae in an aquarium, by marrying up your nitrate and phosphate readings. In the table, there are really only three lines relevant to the saltwater hobby – the top three.

When keeping a marine tank, a reefer will ensure that their phosphate level does not rise above 0.1ppm (mg/l). With this in mind, you can see on the table that if you were to have phosphate of 0.1ppm, then the ideal balanced ratio for nitrate should be 15ppm.

History behind the Redfield Ratio

The Redfield Ratio was named after a discovery by an Oceanographer called Alfred C. Redfield in 1934. He was studying marine biomass and plankton across all the ocean regions including Atlantic, Indian, Pacific oceans and Barents Sea and comparing results he found with previous published research.

Alfred found that the N:P (Nitrogen: Phosphorous) ratio remained consistent across all oceanic bodies of water, in all marine organic matter (living and dead). A ratio of 16:1.
The full ratio was found to be C:N:P (carbon, nitrogen and phosphorous) 106:16:1.

Sometime later a man by the name of Adriaan Briene made news by showing the Dutch Aquarium Society that he could solve a blue-green algae problem in his fresh water aquarium by applying the ratio of 1 part phosphate to 16 parts nitrate (after he had thoroughly researched the Redfield Ratio).
To everyone’s amazement, at a time when algae growth meant time for a water change, he balanced the ratio on his aquarium and proved his theory correct.

How do you use the Redfield Ratio in a saltwater tank?

Using the ratio above of 16:1, we know that our systems will be balanced with one part phosphate and 16 parts nitrate.

For example:

If your phosphate reading is 0.03ppm, multiply 0.03 by 16 and the ideal nitrate level should be 0.48ppm nitrate. We can round that off to 0.5ppm.

Should your nitrate read 5ppm, divide 5 by 16, and your ideal phosphate should be balanced at 0.3125ppm. We can round that off to 0.3ppm. This explains why in certain circumstances, reefers can have very high readings, and if they are balanced they tend not to have any unwanted algae problems.

You can work with this ratio between 1:14 and 1:16, wherein your nutrients will be balanced. Generally speaking, going by the Redfield ratio, if you want to remain algae free and maintain phosphate 0.1ppm and under – an accepted level – then your nitrate should be no more than 1.6ppm!

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What happens when the Redfield ratio becomes unbalanced?

There are a number of scenarios that can lead to the unbalancing of the Redfield ratio. In fact, it is quite easy for this to happen in an enclosed system, and the smaller the aquarium is, the more likely it is to occur.

Prolific algae growth due to unbalanced nutrients

Algae such as Green Hair Algae, Cyanobacteria, and Dinoflagellates are all thought to become a problem when nutrients are not kept in check, or the Redfield ratio becomes unbalanced.

Green Hair Algae is known to take a foothold when phosphate increases and becomes unbalanced. Your nitrate may be maintained at a comfortable level, but as the phosphate level increases, the ratio is no longer in place. In fact, the very first thing a reefer will do is test for phosphate and add GFO, hoping that they have caught it before it takes over.

Cyanobacteria is one of those things that seem to happen for no reason, but in my experience I have had it occur twice. Both of those occasions was caused by my nitrate almost zeroing out because of rapidly growing macroalgae in my refugium. According to the Redfield ratio, my nitrate and phosphate reversed ratio.

Dinoflagellates can be caused by lack of both nitrate and phosphate in a new system, or even a lack of either one. In established systems, dinoflagellates can emerge when you have allowed your nutrients to unintentionally become unbalanced for too long. In my case, I was having a problem reading the colour chart on my phosphate test kit. I assumed everything was okay. Then I noticed my coralline algae had stopped growing…the dinoflagellates noticed too! This was another case of the unbalancing of nutrients.

How do you prevent the Redfield ratio from becoming unbalanced?

Knowing what the Redfield ratio is, and knowing how to put it in place are two different things. In order to maintain the ratio we rely on methods of nutrient export, and methods of nutrient introduction. Your system will determine what you need to do keep the ratio balanced.

Keeping phosphate controlled

The priority is to keep your phosphate as low as possible by ensuring that your GFO is correctly maintained. The lower your phosphate is, the better. There should always be a trace amount in your system to be utilised by your inhabitants. Keeping trace amounts of phosphate in your system is fairly easy to achieve, and almost impossible to have none (except in a new setup). I change my media every 4 weeks and this maintains my level at just under 0.03ppm, but it is acceptable to have up to 0.1ppm.

If you find you are having difficulty keeping you phosphate from bottoming out, even when feeding too much, you may need to think about dosing phosphate with a product like Brightwell Aquatics NeoPhos

Keeping nitrate controlled

As mentioned above, it is possible to zero out on nitrates. This normally happens in a system where nutrients are already kept low. In aquariums such as these, a very close eye should be kept on the nitrate level to prevent it from dropping too low. This invariably occurs due to a filtration system which is too efficient. My system is one such system, and I need to ensure that my macro algae is harvested regularly to prevent a substantial drop.

If you do find that nitrate is dropping too low, it may be time to start adding nitrate to your aquarium with a product like Brightwell Aquatics NeoNitro

Brightwell Aquatics NeoPhos - Phosphorus Supplement for Ultra-Low Nutrient Reef Aquarium Systems, 500ml
  • Recommended for advanced reef aquarists for maintaining low nutrient reef aquariums
  • May be used with MicroBacter7 Reef BioFuel NeoNitro or Katalyst to improve health and coloration of inhabitants
  • Lowers ammonia nitrite and nitrate concentrations without the use of chemical filtration media and pollution
Brightwell Aquatics NeoNitro - Nitrogen Supplement for Low Nutrient Reef Aquariums, 500 ml
  • Recommended for advanced reef aquarists for maintaining low nutrient reef aquariums
  • May be used with MicroBacter7, Reef BioFuel, NeoPhos, or Katalyst to improve health and coloration of inhabitants
  • Lowers phosphate concentration without the use of chemical filtration media and pollution

Does maintaining the Redfield ratio work?

I think there is something in it. But as far as maintaining the correct levels manually is concerned, I fall short of that goal. Our home test kits are known to be fairly accurate but nowhere near as accurate as a lab test such as ATI ICP-OES Test Kit. I may well be maintaining my nitrate at the correct level but with home test kits all I get are what I consider guesstimates at times. Given the constraints of our test kits (against high accuracy lab testing) it is difficult to say at any time if you have both levels perfect. However, by continually striving to attain the ratio, I believe that it keeps you attune with your aquarium, instead of hoping for the best without any specific nutrient goals in place.

The Redfield Ratio – Balancing Nutrients in a Saltwater Aquarium
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