Are We Really Chasing Nitrogen or Is Phosphorus the Key?

[CHSUB]

What’s your view with phosphates?

I keep them low, currently .00 to .02 with Hanna but I don’t run any po4 absorbing media and recent ICP test shows .009; clean the filter sock 2 times a week. I feed corals directly daily with pumps off. You need po4 but I don’t rely on hobby test kits to tell me anything, I observe the corals. I have run tanks with po4 at >.5 with great success, however stonies grew at a snails pace and brown color. Better growth and color with low but available po4, imo.

Here is my high po4 and 0 no3 tank from 2003…

 

[CHSUB]

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

If you have fish and they are fed your tank, in almost all cases, has enough nitrogen; any reading on a hobby test kit is unused extra waste.

 

[Randy Holmes-Farley]

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.

I’m not seeing a conundrum.

Nitrate is not a good indicator of N availability, except that if it is not too low, you can be sure there is at least some, even if it’s not the preferred forms.

I do think many folks recommend too little phosphate.

 

[6272862829]

I’m not seeing a conundrum.

Nitrate is not a good indicator of N availability, except that if it is not too low, you can be sure there is at least some, even if it’s not the preferred forms.

I do think many folks recommend too little phosphate.

Randy most people in the hobby base their ammonia and urea dose with nitrates. What is your view on phosphorus

 

[6272862829]

Nitrate is not a good indicator of N availability, except that if it is not too low, you can be sure there is at least some, even if it’s not the preferred forms.

Wait I’m confused with this statement. In this post you stated otherwise.
You stated one indication of low nitrogen is very low nitrates. So how come you changed your mind. Is it and indicator or is not?

 

[Randy Holmes-Farley]

Randy most people in the hobby base their ammonia and urea dose with nitrates. What is your view on phosphorus

I’ve only seen a few folks dose urea, while lots of folks dose ammonia based on various endpoints such as color or raising nitrate.

These are my recommendations:

Randy’s Thoughts on Nutrient Target Ranges

Thoughts on Nutrient Target Ranges by Randy Holmes-Farley Almost nothing in the world of reef aquarium chemistry causes as much discussion as selecting and maintaining ranges for nitrate and phosphate. Some aquarists want low targets, some want...

www.reef2reef.com

From it:

4. What targets seem reasonable? Of course, that depends on all the other factors at play, such as types of corals, availability of ammonia, particulate foods, etc. However, for a mature mixed reef, this would be how I personally would run it:

• Let nitrate float between 5 ppm and 50 ppm. I’d use gentle export in this range, such as growing macroalgae.
• Above 50 ppm, I’d begin to focus more on reducing it, by organic carbon dosing, turf or macroalgae, etc.
• Below 5 ppm, I’d begin to dose ammonia or feed more. The target level might drop lower if dosing ammonia, just like the heavy in/heavy out scenario where nitrate may not be as needed.
• Let phosphate float between about 0.06 ppm and 0.3 ppm. This range is higher than I’ve recommended in the past. I’d use gentle export in this range, such as growing macroalgae.
• Above about 0.3 ppm, I’d begin to focus more on reducing it, by turf or macroalgae, or a binder such as GFO or lanthanum (has its own risks to tangs). If a binder: GO SLOW. Turf and macroalgae will typically be slow enough.
• Below 0.06 ppm, I’d begin to dose sodium phosphate or feed more to get the level up.

 

[6272862829]

I’ve only seen a few folks dose urea, while lots of folks dose ammonia based on various endpoints such as color or raising nitrate.

These are my recommendations:

Randy’s Thoughts on Nutrient Target Ranges

Thoughts on Nutrient Target Ranges by Randy Holmes-Farley Almost nothing in the world of reef aquarium chemistry causes as much discussion as selecting and maintaining ranges for nitrate and phosphate. Some aquarists want low targets, some want...

www.reef2reef.com

From it:

4. What targets seem reasonable? Of course, that depends on all the other factors at play, such as types of corals, availability of ammonia, particulate foods, etc. However, for a mature mixed reef, this would be how I personally would run it:

• Let nitrate float between 5 ppm and 50 ppm. I’d use gentle export in this range, such as growing macroalgae.
• Above 50 ppm, I’d begin to focus more on reducing it, by organic carbon dosing, turf or macroalgae, etc.
• Below 5 ppm, I’d begin to dose ammonia or feed more. The target level might drop lower if dosing ammonia, just like the heavy in/heavy out scenario where nitrate may not be as needed.
• Let phosphate float between about 0.06 ppm and 0.3 ppm. This range is higher than I’ve recommended in the past. I’d use gentle export in this range, such as growing macroalgae.
• Above about 0.3 ppm, I’d begin to focus more on reducing it, by turf or macroalgae, or a binder such as GFO or lanthanum (has its own risks to tangs). If a binder: GO SLOW. Turf and macroalgae will typically be slow enough.
• Below 0.06 ppm, I’d begin to dose sodium phosphate or feed more to get the level up.

But if nitrates is not a good indicator why target those numbers?

 

[Randy Holmes-Farley]

Wait I’m confused with this statement. In this post you stated otherwise.
You stated one indication of low nitrogen is very low nitrates. So how come you changed your mind. Is it and indicator or is not?

Perhaps that would have been better written as:


Some reef aquaria may have chronic low nitrogen availability. If nitrate is very low, that may indicate inadequate N availability. Nitrate dosing is a fine remedy, but it is possible that ammonia dosing may be more beneficial for some organisms since ammonia can be energetically easier to assimilate than is nitrate.

 

[Randy Holmes-Farley]

But if nitrates is not a good indicator why target those numbers?

Because it is insurance that N is adequate, regardless of the availability of other N sources that are hard to measure.

The article makes these issues clear (I hope).

 

[6272862829]

Because it is insurance that N is adequate, regardless of the availability of other N sources that are hard to measure.

The article makes these issues clear (I hope).

The what I wrote makes it clear also right ? I’m having a hard time understanding you first reply now.

 

[ThisIsTheWay]

The what I wrote makes it clear also right ? I’m having a hard time understanding you first reply now.

The way I understand it, Randy is saying that low nitrates doesn't mean there isn't nitrogen in your system. You might have very low nitrates but plenty of nitrogen. In this case, low nitrates is not a good indicator of available nitrogen in the system.
HOWEVER. If you do have elevated nitrates, then you can be sure you have at least some nitrogen since nitrate is a source of nitrogen for corals and organisms in your tank, even if it's not their preferred form of getting nitrogen.

@Randy Holmes-Farley, feel free to correct me if this is wrong.

 

[6272862829]

The way I understand it, Randy is saying that low nitrates doesn't mean there isn't nitrogen in your system. You might have very low nitrates but plenty of nitrogen. In this case, low nitrates is not a good indicator of available nitrogen in the system.

In this case this makes ammonium bicarbonate dosing base of nitrates not relevant. Basically people are blind dosing.

 

[6272862829]

The way I understand it, Randy is saying that low nitrates doesn't mean there isn't nitrogen in your system. You might have very low nitrates but plenty of nitrogen. In this case, low nitrates is not a good indicator of available nitrogen in the system.
HOWEVER. If you do have elevated nitrates, then you can be sure you have at least some nitrogen since nitrate is a source of nitrogen for corals and organisms in your tank, even if it's not their preferred form of getting nitrogen.

@Randy Holmes-Farley, feel free to correct me if this is wrong.

It would be interesting to see how does a tank that is heavily dosed with ammonium bicarbonate looks like in an NDOC test. I do have what a urea/ammonium/nitrates dosed tank look like under NDOC

 

[reefluvrr]

Hi @.AcroKiller.
I personally feel phosphorus is more important than nitrogen in my mixed reef tank.
Maybe @Hans-Werner can help correct anything I may say wrongly.

About 5 years ago, I dosed nitrate in hope of getting better sps colors. In my experience, it made no appreciable difference in color or growth when I had 7 to 15ppm NO3 versus 0 ppm NO3.

Then about 1.5 years ago, I started to dose Ammonium bicarbonate. Initially I would have 0 ppm NO3 and slowly dosed Ammonium Bicarbonate daily until I can see my NO3 go up to about 3ppm.
I think by dosing this way, when I saw my NO3 reach up to a measurable number, it would mean my mixed reef system can not take up additional ammonium/ Nitrogen and now NO3 is the resultant biproduct. Again, I did not see better colors. I did have to clean my glass more frequently when my Nitrogen levels go up though. I honestly cannot say anything about improved growth from dosing ammonium as I never had fast growing corals to begin with. I felt I had good colors even without dosing ammonium, I therefore stopped dosing it for about 1 year now.

After Salem Clemens talked about dosing urea as an organic source of nitrogen, I went down the rabbit hole again. Now I am dosing 75% urea and 25% ammonium bicarbonate. I have been dosing this for daily for 2 weeks now. So far I still have 0ppm NO3 reading on my Hanna meter. I once again have to clean my glass more frequently, but one noticeable change may not be all too positive. My sps colors appeared to be more brown...
It is still too early to say anything positive or negative with urea dosing for nitrogen, but wanted to give you my experience thus far.

As far as phosphorus goes, my phosphate level is about 0.05 currently. I do carbon dose to keep my PO4 levels under 0.1. If I do not carbon dose, my PO4 levels will go above 0.3ppm. I find carbon dosing helps convert measurable inorganic phosphate to unmeasurable organic phosphate. I think this just means I still have the same amount of phosphorus, but corals uptake organic phosphate to grow.

Anyone, please correct or share your experience regarding what I just written. Thanks

 

[hatfielj]

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.

New study documents increased growth of staghorn Acropora in higher phosphate levels | Reef Builders | The Reef and Saltwater Aquarium Blog

Acropora corals are a mysterious creature with complex needs and one of those that we thought we’d nailed long ago was their “need” for minimal phosphate concentration in the water. A new study by…

reefbuilders.com

Fast forward 14 years from when that study was published, do you have any thoughts now that help explain this unexpected finding? Specifically, I wonder if it had more to do with the type of lighting available in 2011 compared to today (MH vs LED)? Our tanks are quite different now then they were 14 years ago in many ways, but especially lighting.

 

[CHSUB]

my mixed reef system can not take up additional ammonium/ Nitrogen and now NO3 is the resultant biproduct

I agree. It is undisputed that corals need nitrogen and phosphorus, much of the debate among hobbyists is the amount. Imo, the reliant on hobby test kits is part of the problem both with what we can test and the accuracy and resolution of test kits. Nitrogen specifically is only confirmed with the end product no3, ignoring all the other sources of nitrogen. Hobbyists should first rely on eye test. My tank test 0 with hobby testing and many predict disaster and/or dinoflagellate which is laughable. Fed healthy fish and corals and low to unreadable inorganic nutrients with hobby test kits is most desirable.

 

[ellisbart]

This talk by Charles Delbeek is really interesting and insightful into the relationships between nitrogen and phosphorous - which is significantly more important than the levels themselves.

 

[Randy Holmes-Farley]

The way I understand it, Randy is saying that low nitrates doesn't mean there isn't nitrogen in your system. You might have very low nitrates but plenty of nitrogen. In this case, low nitrates is not a good indicator of available nitrogen in the system.
HOWEVER. If you do have elevated nitrates, then you can be sure you have at least some nitrogen since nitrate is a source of nitrogen for corals and organisms in your tank, even if it's not their preferred form of getting nitrogen.

@Randy Holmes-Farley, feel free to correct me if this is wrong.

Yes, exactly. :)

 

[Randy Holmes-Farley]

This talk by Charles Delbeek is really interesting and insightful into the relationships between nitrogen and phosphorous - which is significantly more important than the levels themselves.

I do not believe the ratios are particularly important as long as you have a sufficient amount of both. :)

 

[Randy Holmes-Farley]

Hi @.AcroKiller.
I personally feel phosphorus is more important than nitrogen in my mixed reef tank.
Maybe @Hans-Werner can help correct anything I may say wrongly.

About 5 years ago, I dosed nitrate in hope of getting better sps colors. In my experience, it made no appreciable difference in color or growth when I had 7 to 15ppm NO3 versus 0 ppm NO3.

Then about 1.5 years ago, I started to dose Ammonium bicarbonate. Initially I would have 0 ppm NO3 and slowly dosed Ammonium Bicarbonate daily until I can see my NO3 go up to about 3ppm.
I think by dosing this way, when I saw my NO3 reach up to a measurable number, it would mean my mixed reef system can not take up additional ammonium/ Nitrogen and now NO3 is the resultant biproduct. Again, I did not see better colors. I did have to clean my glass more frequently when my Nitrogen levels go up though. I honestly cannot say anything about improved growth from dosing ammonium as I never had fast growing corals to begin with. I felt I had good colors even without dosing ammonium, I therefore stopped dosing it for about 1 year now.

After Salem Clemens talked about dosing urea as an organic source of nitrogen, I went down the rabbit hole again. Now I am dosing 75% urea and 25% ammonium bicarbonate. I have been dosing this for daily for 2 weeks now. So far I still have 0ppm NO3 reading on my Hanna meter. I once again have to clean my glass more frequently, but one noticeable change may not be all too positive. My sps colors appeared to be more brown...
It is still too early to say anything positive or negative with urea dosing for nitrogen, but wanted to give you my experience thus far.

As far as phosphorus goes, my phosphate level is about 0.05 currently. I do carbon dose to keep my PO4 levels under 0.1. If I do not carbon dose, my PO4 levels will go above 0.3ppm. I find carbon dosing helps convert measurable inorganic phosphate to unmeasurable organic phosphate. I think this just means I still have the same amount of phosphorus, but corals uptake organic phosphate to grow.

Anyone, please correct or share your experience regarding what I just written. Thanks

I don’t disagree that phosphate is very important, and it does not have simple dissolved substitutes the way nitrate has ammonia and urea. Thus, P may seem more important than N, even when both are needed.