Are We Really Chasing Nitrogen or Is Phosphorus the Key?

[6272862829]

Are We Really Chasing Nitrogen or Is Phosphorus the Key?

In the reefing world, the conversation around nutrient management often centers on nitrate and phosphate levels. But after observing two very different systems over time, I’m beginning to question the traditional focus, are we really chasing the right form of nitrogen, or should we be paying more attention to phosphorus?

Let’s take a look at two reef tanks that have offered some interesting insight:

Tank 1: The Nutrient Paradox

This is an Innovative Marine 25 Lagoon system, holding about 20 gallons of total water volume. It’s a mixed reef with SPS, LPS, and soft corals. Despite being a classic example of a nitrate-deficientsystem where nitrate consistently registers zero, this tank is phosphate-rich, often maxing out the Hanna Checker, though levels occasionally dip to around 0.20 ppm.

Here’s the surprising part: the colors and coral growth in this tank have always been phenomenal. No nitrogen dosing, no nitrate traceability yet the results speak for themselves.

Tank 2: The Nitrogen-Targeted Approach

Now compare that to a 100-gallon SPS/LPS system. This tank has low nutrient levels across the board, with nitrate hovering around zero and phosphate typically between 0.03 and 0.06 ppm. To compensate, it receives a blend of urea, ammonia, and nitrate to elevate nitrate levels to about 3–5 ppm.

But even with this careful nutrient supplementation, the tank’s color and growth remain just okay. There’s no significant improvement despite targeted nitrate increases.

So What Gives?

If nitrogen alone were the missing piece, Tank 2 should be thriving. Instead, it’s the nutrient-skewed, phosphate-heavy Tank 1 that’s outperforming. This leads to a serious question:

Is it nitrogen we need to chase, or is phosphorus playing a more critical role than we’ve been led to believe?

While every reef system is different, these observations suggest it’s time to rethink the standard nutrient dogma. Perhaps the interplay between nitrogen sources and how corals and beneficial bacteria process them matters more than the numbers alone. Or maybe, in some systems, elevated phosphate isn’t the villain it’s made out to be.

One thing’s clear: the conversation around nitrate, urea, ammonia, and phosphate isn’t as black and white as it seems. Sometimes, the tank tells a different story.

Please comment with your view on this.

 

[hatfielj]

A lot of people only monitor Phosphates...POTO is a good example. The specific nitrate level doesn't really matter as long as it's not 0 and as long as it's not too high (like less than 25 for instance). Alk and Phos are the two we have the most control over and the 2 that are most likely to kill corals if they quickly change.

 

[bubbgee]

A lot of people only monitor Phosphates...POTO is a good example. The specific nitrate level doesn't really matter as long as it's not 0 and as long as it's not too high (like less than 25 for instance). Alk and Phos are the two we have the most control over and the 2 that are most likely to kill corals if they quickly change.

I’m with this approach as well. I keep my eye on alk and phosphates. Now I need to dial in my trace so colors are supposedly better. It’s my next experiment.

 

[JD10]

A lot of people only monitor Phosphates...POTO is a good example. The specific nitrate level doesn't really matter as long as it's not 0 and as long as it's not too high (like less than 25 for instance). Alk and Phos are the two we have the most control over and the 2 that are most likely to kill corals if they quickly change.

I agree with this approach.

Contrary to everything I knew at the time, Acro growth took off under high phosphate levels (0.5 vs 0.2, 0.09) in my study. I was expecting them to be dead or growth greatly stunted at least.

https://reefbuilders.com/2011/12/05/acropora-phosphate-growth/

 

[obsessedfishlady]

A lot of people only monitor Phosphates...POTO is a good example. The specific nitrate level doesn't really matter as long as it's not 0 and as long as it's not too high (like less than 25 for instance). Alk and Phos are the two we have the most control over and the 2 that are most likely to kill corals if they quickly change.

I agree with this approach.

Contrary to everything I knew at the time, Acro growth took off under high phosphate levels (0.5 vs 0.2, 0.09) in my study. I was expecting them to be dead or growth greatly stunted at least.

https://reefbuilders.com/2011/12/05/acropora-phosphate-growth/

Very interesting. Especially when people bash you if you’re phosphates are even the tiniest bit elevated. I always struggled to keep acros alive but my phosphates were always sorta on the low side as an attempt to keep them under control.

 

[MnFish1]

This is interesting - I wonder though if what you're seeing can be generalized to all types and varieties of corals (soft, LPS, SPS) as well as all tanks (new vs established, different light, etc, etc). I do not think you can. In my experience what tends to happen is that people get a coral - it either does well or it doesn't - and in the end one has a tank full of corals and fish that can coexist within the boundaries of the conditions in that particular tank.

I'm not a big believer in the common wisdom that nutrient levels are the prime driver of success - since many people have tanks that are successful with high nitrates and low PO4, and any variation thereof. But it's an interesting thought.

 

[MnFish1]

Here is an interesting article I remember reading from before - I think the take home message is that corals grow well with higher PO4 - but they are less 'healthy'
https://www.sciencedirect.com/science/article/abs/pii/S0022098111004588

 

[6272862829]

I agree with this approach.

Contrary to everything I knew at the time, Acro growth took off under high phosphate levels (0.5 vs 0.2, 0.09) in my study. I was expecting them to be dead or growth greatly stunted at least.

https://reefbuilders.com/2011/12/05/acropora-phosphate-growth/

Thank you for sharing this article.

 

[6272862829]

Here is an interesting article I remember reading from before - I think the take home message is that corals grow well with higher PO4 - but they are less 'healthy'
https://www.sciencedirect.com/science/article/abs/pii/S0022098111004588

Thank you for sharing this article.

 

[bubbgee]

A lot of people only monitor Phosphates...POTO is a good example. The specific nitrate level doesn't really matter as long as it's not 0 and as long as it's not too high (like less than 25 for instance). Alk and Phos are the two we have the most control over and the 2 that are most likely to kill corals if they quickly change.

I agree with this approach.

Contrary to everything I knew at the time, Acro growth took off under high phosphate levels (0.5 vs 0.2, 0.09) in my study. I was expecting them to be dead or growth greatly stunted at least.

https://reefbuilders.com/2011/12/05/acropora-phosphate-growth/

My experience is the reverse, lower alk uptake on higher phosphate.

Is it possible that it’s a biome issue that spurs that difference?

 

[JD10]

Here is an interesting article I remember reading from before - I think the take home message is that corals grow well with higher PO4 - but they are less 'healthy'
https://www.sciencedirect.com/science/article/abs/pii/S0022098111004588

Yes, this is my study. We proposed that coral growth alone may not always be a reliable indicator of overall health, as specimens exposed to elevated phosphate levels appeared more porous and potentially more susceptible to breakage during high-energy storm events.


However, this would likely not pose a concern in a controlled reef aquarium environment. Additionally, no noticeable differences in polyp extension were observed across the three experimental tanks.

 

[CHSUB]

I’m beginning to question the traditional focus, are we really chasing the right form of nitrogen

Traditional focus? Intentionally elevating no3 is not “traditional” it’s a new thing that is based on myth, hype, and false hobby observations. Imo, Ime, in a fed tank with fish there is no reason to have detectable no3 with a hobby test kit.

 

[JD10]

Very interesting. Especially when people bash you if you’re phosphates are even the tiniest bit elevated. I always struggled to keep acros alive but my phosphates were always sorta on the low side as an attempt to keep them under control.

I felt a huge sense of relief when I realized phosphate wasn’t the “boogeyman” I once believed it to be. That said, I do think there can be noticeable variation depending on the species and specific system setup.

 

[6272862829]

Traditional focus? Intentionally elevating no3 is not “traditional” it’s a new thing that is based on myth, hype, and false hobby observations. Imo, Ime, in a fed tank with fish there is no reason to have detectable no3 with a hobby test kit.

What’s your view with phosphates?

 

[MnFish1]

I felt a huge sense of relief when I realized phosphate wasn’t the “boogeyman” I once believed it to be. That said, I do think there can be noticeable variation depending on the species and specific system setup.

However, with that one study, in which the conclusion was that the coral was 'stressed and not healthy', it's completely impossible to say that PO4 is or is not a bogeyman. As @Thales has shown a high PO4 level is not necessarily 'bad'. What is not being shown is whether low nitrates are 'bad' or with different alkalinities, lighting, etc - higher PO4 may or not be better or worse for coral and aquarium health.

 

[JD10]

However, with that one study, in which the conclusion was that the coral was 'stressed and not healthy', it's completely impossible to say that PO4 is or is not a bogeyman. As @Thales has shown a high PO4 level is not necessarily 'bad'. What is not being shown is whether low nitrates are 'bad' or with different alkalinities, lighting, etc - higher PO4 may or not be better or worse for coral and aquarium health.

I spent a lot of time and money up to that point trying to keep phosphate at 0. For me personally, it was a relief to know that the wheels weren’t coming off if levels rise a bit.

 

[Tony Thompson]

Maybe take into account Nitrogen , rather than just looking at the reading for HR Nitrate on a Hanna. There is more than one source of nitrogen for your corals than nitrate. In fact the metabolism for other forms of nitrogen may be simpler and require less effort on behalf of the coral.

The building blocks of life are summarised as Nitrogen, Phosphorus, Carbon, Sulphur, Oxygen and Hydrogen.

 

[Thales]

However, with that one study, in which the conclusion was that the coral was 'stressed and not healthy', it's completely impossible to say that PO4 is or is not a bogeyman. As @Thales has shown a high PO4 level is not necessarily 'bad'. What is not being shown is whether low nitrates are 'bad' or with different alkalinities, lighting, etc - higher PO4 may or not be better or worse for coral and aquarium health.

I think my first talk on this was in 2012, and the first article was in 2014. :D

 

[obsessedfishlady]

Very interesting. Especially when people bash you if you’re phosphates are even the tiniest bit elevated. I always struggled to keep acros alive but my phosphates were always sorta on the low side as an attempt to keep them under control.

I felt a huge sense of relief when I realized phosphate wasn’t the “boogeyman” I once believed it to be. That said, I do think there can be noticeable variation depending on the species and specific system setup.

I’ll have to keep this in mind. I have a bad bad habit of chasing numbers (though I had a beautiful tank as a result) and it’s extremely stressful and makes the hobby less enjoyable at times. I’m trying to be better about not chasing. Ryan also said what another person mentioned - that people buy coral and it does well or or doesn’t - he pretty much said the same thing. That a healthy coral can adjust to almost any (within reason) parameters.

 

[6272862829]

Maybe take into account Nitrogen , rather than just looking at the reading for HR Nitrate on a Hanna. There is more than one source of nitrogen for your corals than nitrate. In fact the metabolism for other forms of nitrogen may be simpler and require less effort on behalf of the coral.

The building blocks of life are summarised as Nitrogen, Phosphorus, Carbon, Sulphur, Oxygen and Hydrogen.

How would one take account nitrogen. Most of use rely on a nitrate reading to have a piece of mind that there’s nitrogen been supplemented in the system. For example the supplement I dose is 50% urea 25% ammonia and 25% nitrate. So relying on a nitrate reading gives me the assurance on how much to dose