Nitrate, phosphate, and redfield ratio

Discussion in 'Reef Chemistry' started by rygh, Dec 9, 2014.

  1. rygh

    rygh Supporting Member

    As usual, going to a talk by Rich Ross gets me thinking and reading ...

    First as background, the Redfield Ratio is the the atomic ratio of carbon, nitrogen, and
    phosphorous found in phytoplankton. In itself, mostly a fun science data point.
    Roughly C:N:p = 106:16:1

    The theory beyond that is how your aquarium N/P ratio affects competition between life forms.

    To perhaps badly over simplify my understanding:
    * If the water has roughly that ratio, phytoplankton and a lot of marine life, presumably including
    coral, do really well. Makes sense.
    * If the N/P ratio falls, meaning excess phosphate, Blue Green Algae and Cyano thrive, and can
    out-compete other marine life. They can fix more Phosphate.
    * If the N/P ratio rises, meaning excess nitrate, Green/Hair Algae thrive, and can out-compete
    other marine life. They can fix more Nitrate.

    So what you get is this table:

    So the basic idea is that chasing low-phosphates or low-nitrates by themselves can do as much harm as good.
    What you care about more is the ratio.
    Interestingly, the larger the numbers, the easier it looks to control the ratio.

    However, there is another factor about general nutrient levels being a problem.
    Very high levels of nutrients seem to overwhelm the equation, plus nitrates can harm fish.

    So interestingly, a sweet spot might be around 15 ppm Nitrate, and 1.5 ppm phosphate.
    Sounds a bit familiar.....

    So that is the introduction to what might be a fun discussion.

    Original table from here:


    Other things I will try to post soon, when I have time:
    * I am doing a Nitrate/Phosphate experiment on my tank.
    * An article that Herbivores may directly help self-regulate that ratio
    * Algae Scrubbers give a unique perspective and experiment on this
  2. bluprntguy

    bluprntguy Webmaster

    All the studies I've seen seem to indicate to me that the only way to slow the growth of microalgae is to limit either phosphate, nitrate, or iron. In most aquariums it's just easiest to limit INORGANIC phosphate by running GFO. This simply starves the algae of just one of the many things that it needs to thrive. Limiting phosphate too much obviously impacts other organisms that also need phosphate, so the idea is that you should keep it low enough to slow algae growth, but not too low to impact other organisms.

    You seem to be suggesting that as long as you keep nitrate and phosphate in proportion to the Redfield Ratio you can raise them (within reason) and you won't get nuisance algae or bacteria. I don't follow this thought process at all. Providing available nitrate and phosphate (and all the other required elements) in ideal proportions for marine organisms to use (the "Redfield Ratio") should theoretically result in large growth rates for everything in your tank, including the nuisance algae. If a tank has elevated nitrate and phosphate levels, and doesn't have nuisance algae, then there is a lack of something else that is limiting the growth of algae.

    The Redfield Ratio was developed circa 1930-something. More recent studies have pretty much debunked it's accuracy finding instead that the ratio is highly variable. Given the questions on the Redfield ratio's accuracy, our inability to test for inorganic phosphate very well, and our complete inability to test at all for organic phosphate, it seems like chasing the Redfield Ratio would be an impossible task anyway.
    Last edited: Dec 10, 2014
  3. rygh

    rygh Supporting Member

    First, I am not really pushing the theory as much as hopefully starting a fun discussion.

    Yes, perfect proportions would generate growth of everything including algae.
    Key though: SOMETHING will limit growth eventually.
    And you want that something to be where Coral may win.

    Put another way:
    If your limit is Low-Nitrate : Blue-Green Algae and Cyano win, and take over.
    If your limit is Low-Phosphate : Std Green Hair Algae thrives, and takes over.
    If your limit is something else, Coral might have an edge.

    And we give Coral that edge.
    We put snails and herbivores in the tank, which gives
    a huge advantage to coral, since it is not being eaten all the time.

    Physical space in the light can be another win.
  4. rygh

    rygh Supporting Member

  5. jonmos75

    jonmos75 Supporting Member


    I have been taught this by a very successful refer and he taught me this and what I strive for and I believe that it works....I gave the Redfield ratio to Nav on his thread of "Please Help! (tank maybe crashing)"

    Don't get me wrong you wouldn't want high Phosphates or Nitrates, but as long as they are balanced in the ratio then you wont get algae breakouts.

    As of my test this weekend I had:

    0.6 N03
    0.03 PO4
    and have 265 Liters (

    so here are screen shots of that it says in German and then I copy and past over to Google Translator and this is what it tells me about my ratio....It say that my phosphates are out of balance and have a tendency for green algae which is what I am getting on my glass but my ratio is close to being balanced so the green algae is not bad at all.

    So I am a BIG believer in this Ratio...(but that is my opinion)

    Attached Files:

  6. Enderturtle

    Enderturtle Volunteer

    I am currently in a seemingly hopeless battle with green hair algae at the moment and I think it is either surviving off of very low nutrients or my liverocks are leaching nutrients.

    Am interested in learning more about carbon dosing. Wonder if it will work against this hair algae.
  7. rygh

    rygh Supporting Member

    Any idea on your Nitrate and Phosphate levels?
    It would be curious to calculate the ratio.

    If the above theory is correct, you need to reduce nitrates, but not necessarily phosphates.

    Carbon/Vodka dosing is a pretty common way to reduce nitrates, yes. Not sure what it does
    to phosphates.
    Key: Read up on it first. And take it SLOW. Do not add the full dose at first.
  8. HiFidelity

    HiFidelity Guest

    Carbon dosing does nothing for Phosphates, I'm up to 45ml vinegar & 10ml vodka weekly (50 gal tank) and algae still grows, I think finally I'm going to give in & run GFO.

    I've been doing some reading on other people's experiences and it appears that algae scrubbers DESTROY phosphates, when configured/sized/run properly.
    Another member here has been running a large algae scrubber on his tank for quite some time now, he feeds generously, hardly does water changes and I could not find algae anywhere in his tank.

    I gave up on testing since no kit that I've owned registers a reading with my tank on either nitrates or phosphates except for the low range Hanna Phosphate checker but it's always above the tester's range and bellow all other tests, though I do not own the regular Hanna Phosphate checker.

    I found the info in this thread to be quite interesting and really would love to give this ratio a shot, but it's like shooting fish in a barrel if I can't get a reading on the test kits.
  9. bluprntguy

    bluprntguy Webmaster

    Normal carbon dosing (vodka, vinegar, sugar) tends to reduce nitrates faster than it reduces phosphates in most aquariums (see Redfield Ratio). If you reach a zero point on nitrates and you still have phosphates, you can just dose your tank with nitrate. Once you have some nitrate in your tank again, bacteria and other organisms start growing and use up both nitrate and phosphate, which will eventually bring down your overall phosphates.

    The zeovit system actually includes nitrates in one of the supplements to accomplish this. If you are just doing standard vodka dosing, there are planted tank additives that can be used to increase the nitrate.

    Or you can just run GFO...
  10. rygh

    rygh Supporting Member

    Regarding my personal experience with Algae Scrubbers:

    It tends to support some of this theory.

    I was fighting endless hair algae as well. Had high nitrates and phosphates.
    I added an Algae Scrubber.
    Hair algae problems are gone. Nitrates are gone.
    Phosphates are OK, as an absolute value
    But I battle Cyano pretty hard, which is arguably worse than hair algae.

    Algae Scrubbers are very good at removing Nitrates. A fact among a lot of fiction. :rolleyes:
    They also removes some phosphates of course, but the ratio is that of Algae, not Phyto/Redfield.
    So N/P is much larger. Somewhere between 50/1 to 100/1, far from 16/1. Not sure on real number.
    The food I add is a mixture of seaweed and meaty foods.
    The seaweed N/P I feed is likely identical to the scrubber. The meaty food has a lower N/P.

    So as a result, I have undetectable Nitrates, but slowly rising phosphates.
    My normal N/P ratio is 0(undetectable, not really 0) / 0.04 or so normally. Very low N/P.
    Numbers most people would consider pretty good.

    Yet I fight Cyano, which was always perplexing.

    Adding GFO definitely helps.
  11. HiFidelity

    HiFidelity Guest

    My assumption too is that my nitrates have bottomed out and phosphates are still there, I've started phasing out the vodka since my fuge has now taken off and has been running for over a month, can't just stop vodka all at once so I'm reducing dose by roughly 1ml/week. I would like to see how the fuge fairs on its own without carbon dosing but I'm keeping my vinegar because I'm spiking my kalkwasser with it so I guess fuge/vinegar is going to be long term.

    I have both Cyano & Green Hair Algae right now, sometime Cyano will die & melt away sometimes it will appear in new places it's strange. GFO once vodka is completely out of the equation as I like to stagger any changes I make on my tank so that the results are more likely to be measurable and more accurate assessment if an improvement/change comes forth.

    That being said, any recommendations for most accurate test/kit to measure nitrates? I noticed hanna doesn't make one, I think I have one from almost all of the brands haha but I have not had the time to go through them all in one sitting to compare. Furthermore it turns out I have the Ultra Low Phosphate checker and I am going to need to buy the "Low" range checker if I want to experiment with my N/P ratios :)
  12. bluprntguy

    bluprntguy Webmaster

    I use Salifert. If you have low nitrates you read it sideways to get down to lower readings. Seems to work for me.

    Hanna makes two "phosphate" checkers. The "Low Range Phosphate Checker HI713" and the "Ultra Low Range Phosphorous Checker HI736". The Low Range Phosphate Checker spits back readings in ppm (which mosts aquarists understand) and the Ultra Low Range Phosphorous Checker spits back readings in ppb (which people that are involved with hydroponics and pot growing understand). The accuracy of the two units is practically the same. If you already have the Ultra Low Range Phosphorus Checker, you just need to convert the readings. I wouldn't buy a new unit.
    Last edited: Dec 10, 2014
  13. rygh

    rygh Supporting Member

    +1 on keeping the ULN phosphate checker. I have one.
    Just multiply by 3 and divide by 1000.

    Of course .... now we are talking about chasing numbers with fancy equipment.
    That is often a bad sign.:(
  14. jonmos75

    jonmos75 Supporting Member

    My Tests kits are as Followed:

    Red Sea Reef Foundation Pro Test Kir (Ca, Alk, Mg)
    Red Sea Algae Control Test Kit (NO3 PO4)
    Red Sea Colors Pro Test Kit (I2, K, Fe)
    Hanna Checker Alk (check against Red Sea Test Kit) Just for comfort
    Hanna Checker Ultra Low Phosphorous Test (Check Against Red Sea Test Kit) with the Calculation of Phosphorous level (times) 3.066 (divided by)1000= Phosphate level)

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