beananimalUnderstanding the Redfield ratio is easier than you think
The Redfield ratio has become one of the most confidently repeated misconceptions in the reef hobby, and as usual, my mission is to correct the record and stem the nonsense. Marine organisms maintain consistent elemental ratios in their tissues — but that doesn’t mean your water column needs to match them. This simple buffet analogy will help make that obvious.
The Reef Nutrient Buffet
Think of a meal as one unit of the Redfield ratio. Each meal consists of one burger, 16 fries, and 106 ml of Coke. In this analogy, the burger represents phosphorus, the fries represent nitrogen, and the Coke represents carbon.
You eat exactly two of these meals per day — two burgers, 32 fries, 212 ml of Coke. Your friend eats the same meal in the same proportions. Look at the tissues of many marine organisms and you’ll find roughly this ratio: 1:16:106. That’s the Redfield ratio — phosphorus, nitrogen, and carbon. It reflects how organisms are built, not what the water around them contains.
Now look at the buffet. Fries and Coke are stacked floor to ceiling — there’s never a shortage. Burgers, though, are expensive and in short supply. If there are no burgers, you don’t eat, regardless of how many fries and Cokes are sitting there. The burger is your limiting nutrient. You won’t substitute. You won’t eat 20 fries and skip the burger. The ratio of what you consume is fixed by your biology, not by what’s on the table.
Here’s the critical point: nobody needs to arrange the buffet in a 1:16:106 ratio. The buffet just needs enough burgers to go around. Whether there are 10,000 fries or 32 fries doesn’t change how many burgers you eat or how you use them. The supply ratio is irrelevant as long as no single item runs out.
The Reef Tank
The Redfield ratio describes organism tissue composition — what gets incorporated, not what needs to be present in the water column at any particular proportion. Your corals and algae will maintain whatever ratio their biology demands regardless of the N:P ratio in your water, provided neither is truly exhausted.
Our concern in a reef aquarium is absolute concentration, not proportion. Elevated phosphorus inhibits calcification. Elevated nitrogen fuels nuisance algae and cyanobacteria. Each nutrient has its own threshold for causing problems. Those thresholds have nothing to do with each other’s concentration. The question is never “is my N:P exactly 16:1?” It’s “is phosphorus high enough to impair skeletal growth, or is nitrogen high enough to feed algae?”
The other end of the scale matters too. If either nutrient is at or near zero, that’s a limitation problem — organisms can’t meet their biological demands regardless of how much of the other nutrient is present. That also has nothing to do with ratio. It’s simply a shortage.
The Defective Deflective Hedge
To avoid being wrong, some will concede that an exact 16:1 ratio isn’t necessary but argue that staying close to it is still good practice. It’s a hedge that’s the same error with softer language. The underlying mechanism doesn’t operate on ratio. “Close to the ratio” therefore provides no benefit over any other combination of concentrations that keeps both nutrients out of the problem zones.
The hedge survives because it sounds like a reasonable middle ground between strict ratio adherence and dismissing Redfield entirely. It isn’t. It’s the same misapplication in disguise. Randy Holmes-Farley’s work on nutrient target ranges at Reef2Reef is a solid starting point for understanding what those thresholds actually are and why they matter independently of each other.
It Gets Worse
The kicker is that hobbyists who believe they are targeting the Redfield ratio are not even doing the math correctly. The Redfield ratio is a molar ratio — not parts per million. Someone targeting 16 ppm nitrate to 1 ppm phosphate is running a molar ratio closer to 25:1, not 16:1. So not only are they chasing a number, they are chasing a different number than the the actual Redfield ratio!
And Even Worse!
The Redfield ratio is expressed in total nitrogen and total phosphorus. But, we don’t measure total nitrogen or total phosphorus in our systems. Our test kits only measure the inorganic fraction of those nutrients. The dissolved organic nitrogen, dissolved organic phosphorus and any particulate forms are not measured. So, not only are the Redfield adherents chasing the wrong number, they are measuring the wrong thing, by a large margin.
Who Started this Nonsense?
This isn’t obscure fringe advice that bubbled up from anonymous forum posts. It appears in respected reef literature authored by some of the most respected names in the hobby and is therefore repeated by other credible voices. That’s how a flawed application of an oceanographic observation becomes conventional reef wisdom.
Credible source. Wrong conclusion. The mechanism doesn’t support it regardless of who is recommending it.
Targeting Redfield in Reefing is Dead!
At every level the entire concept of targeting Redfield ratios for N and P levels fails. It is the wrong concept to begin with, coupled with the wrong math and the wrong thing being measured. To put it mildly: It is absolute and utter nonsense any way you slice it. The only thing holding it all together is repetition.
-BeanAnimal
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