Randy’s Thoughts on Nutrient Target Ranges

Randy Holmes-Farley

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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 high targets, some want a particular ratio, and others just want to know what the heck is going on. Then comes the actions, with many people wanting to raise N and P and many wanting to lower them, and to do either one, aquarists need to wade through a wide range of possible actions. There are great tanks with very low nutrient levels, and there are great tanks with very high nutrient levels. We should not necessarily assume, however, that the same tank would work as well at both extremes of the ranges.

I do not have any magic recipe, or target ranges. While I will give some suggested target values at the end this article, I mostly want to address why so many aquarists have so many different ideas of what is best in their system. In other words, is it rooted in the science of nutrient uptake by corals?

Figure 1. A very high nutrient system maintained by Richard Ross, with nitrate sometimes above 100 ppm and phosphate above 1 ppm. Details in this link:

1738594426092.png



As we get into reasons why aquarists may have different opinions on what works best in their systems, let’s skip over those opinions that come just from reading somewhere what is best. They may be right or wrong, but it’s not so interesting to try to dissect them. Let’s instead focus on real reasons why a given reefer might correctly have an idea what is best in his or her system, and why that correct opinion may differ greatly from that of another aquarist. This information may help reefers focus in on what is best in their system, independent of what anyone else recommends.

1. A first, and very obvious, reason why reefers may have a dispute on what levels are best may be because they keep creatures with different abilities to extract what they need from the same levels of nitrate and phosphate. It is often stated that “soft corals prefer higher nutrient levels”, but how could that be and why would that be true?

First, is it true that different corals prefer different nutrient levels? The simplistic answer appears to be yes, different species have different abilities to collect nutrients (both in a lab and in the field), although comparative data is very sparse. In the natural environment, the distribution of, for example, soft corals vs acropora sometimes appears to relate to nutrient levels. This linked article shows that in some islands in Indonesia, that the preponderance of soft corals is higher in areas of higher nitrate while acropora dominates in higher phosphate locations. Of course there are other factors, and fingering one alone is impossible. Nevertheless, of the factors examined, nutrients appear to be among the best predictors of coral types at the different locations studied.

https://aquapublisher.com/index.php/ijms/article/html/1476

While this acropora vs soft coral differentiation is strong enough to identify in the wild, getting down the species level in terms of wild abundance at different nutrient locations has not been carefully evaluated. Nevertheless, there’s no reason to assume that corals do not vary significantly in their preferences for different nutrient environments for the reasons discussed in the next few sections.

Both nitrate and phosphate are taken up by corals and other organisms using active transporters on their surfaces exposed to the tank water. These transporters are proteins that span some sort of membrane between the coral body interior and the external environment. They bind phosphate or nitrate on the outside, and dump it inside. All organisms have these, from bacteria to people. Organisms control their uptake by adjusting the numbers and types of these transporters, and such effects have clearly been demonstrated in corals. This linked article, for example, shows such control in uptake capacity in the hard coral Stylophora pistillata.


Indeed, corals that were fed 1 and 3 days before the uptake experiment took up phosphate 42 and 19% slower, respectively, than corals that were fed 21 days before.

…the saturated uptake rate of ammonium increased by 2.5-fold in the presence of 3.0 μmol l–1 of phosphate, thus indicating that the corals or their symbionts were lacking intracellular phosphate to take advantage of the inorganic nitrogen compounds dissolved in their surrounding medium.


There are many such transport proteins, and both the identity and the number of them determine how effectively one organism can take up what it needs relative to another. The upshot is that at, say 0.01 ppm phosphate, one organism may get all it wants while another cannot keep up with its need for phosphate. That is because of the nature and numbers of its uptake transporters in relation to its total need for the transported ion.

The article linked below gives a sense of the complexity and control for phosphate in a single well-studied bacterial system. That level of detailed information is lacking for typical corals we keep.


Nevertheless, there is information on phosphate transport and control mechanisms in corals, such as this article on Stylophora pistillata:


from it:

Our results showed the presence of active phosphate carriers both in the animal and the algal fractions…

Transporters in the animal and the algae presented different affinities for phosphate,…

The velocity of phosphate absorption increased in the light,…

A correlation was found between phosphate uptake rates and the organic or inorganic feeding history of the corals; rates were indeed 4.6 times higher in 8-week starved than in fed corals, and also depended on the repletion status of phosphorus stocks within the symbionts…


The fact that the uptake changed with feeding history means the corals must have changed either the number, or the type, or both, of its phosphate transporters. This result also hints at why it can be very undesirable to lower phosphate too quickly: corals may suffer while they try to ramp up their capability to get what they need from the new, lower level. Likewise with raising nutrients (which seems more of an issue with raising alkalinity too fast, but it’s the same story): corals cannot downregulate their uptake fast enough, and may take inside more of something than is good for them (like a starving person at an all you can eat buffet).

The article linked below even more clearly shows how complicated nutrient uptake can be in corals, with one dinoflagellate symbiont type originally identified from a Montipora verruciosa coral shown to have MANY transporters that it can use, and those may be different than the transporters other corals can use:

https://www.sciencedirect.com/science/article/abs/pii/S0048969720354358

from it:

In addition, this species possesses highly duplicated nutrient transporters including 13 phosphate transporter genes, 62 NO3− transporter genes, and 84 NH4+ transporter genes, which are higher than in other algal species.


Figure 2. A low nitrate system by Reef2Reef member Tusi. When the article was written, he noted phosphate of 0.05 ppm and nitrate of 0.5 ppm. This member may increase bioavailable bacterial foods, which may partly supply some of the N that corals need, by dosing bacteria and carbon dosing.


1738594470475.png


2. In addition to different organisms being used by different aquarists to assess what is best in nutrient terms, there are many different endpoints that one reefer may focus on that are different from another reefer, even assessing the same coral and its local environment. Some of these are:
  • Polyp expansion
  • Growth rate
  • Color
  • Pest algae
  • Pest dinoflagellates
  • Pest cyanobacteria
  • Pest diatoms
  • Others
Each of these has its own world of complexity associated with it. Just picking one at random, polyp expansion, we can easily see how one particular aquarist’s assessment of one particular coral may differ from another aquarist with the same coral if there are differences in lighting, flow, fish picking, other chemicals, pH, etc., that all themselves impact polyp expansion. Even if phosphate or nitrate has an effect on polyp expansion, it may be lost as noise under bigger effects of any of these other factors.

The same sort of variability is true for all of these other endpoints. As another example, the need for available nutrients to prevent dinoflagellate pests in new tanks with bare surfaces may be very different than a mature reef tank with no exposed surfaces where dinoflagellates might take hold. If one way to prevent dino pests is through competition with less troubling organisms occupying the same surfaces, say, bacteria or diatoms or green algae, then that system may benefit from higher nutrients to promote that competition, while other systems without exposed surfaces or with a more mature microbiome may not need those same nutrient levels to avoid dino pests.

3. Another factor that may vary between aquaria is how much N and P a given coral actually needs to grow in a given system. Accepting that factors such as lighting, pH, alkalinity, flow, and trace elements can potentially impact coral growth rates, the rate at which those same corals need to take up N and P will vary with those conditions. Faster growth means it needs more N and P to build new tissues. More need for N and P may mean a different optimal level of nitrate and phosphate in the water to be able to get what it needs. Thus, when one aquarist says 0.03 ppm phosphate gives the best growth, and a second one says no, she sees better growth of the same coral at 0.2 ppm phosphate, the difference could lie in any or all of these and other factors being different between the two systems.

4. Phosphate is known to have a direct and negative impact on hard coral skeleton growth. This linked article, for example, states

… our findings showed that inhibition of coral skeleton formation was due to absorption of phosphate on the skeleton, which inorganically inhibited normal development of juvenile coral skeleton.

https://link.springer.com/article/10.1007/s10126-019-09880-3

The implication of phosphate inhibition of skeletal formation has several implications:
  • Aquarists talking about optimal levels for a hard coral vs soft coral tank may come to different conclusions for this reason alone (perhaps among many others).
  • Aquarists talking about only hard corals may be concerned primarily with growth (perhaps implicating elevated phosphate as bad) while others also referring only to hard corals may be concerned with other endpoints, such as color or polyp extension (which may not have the same implications for elevated phosphate)
5. Nutrient levels can impact reef tank microorganisms, including coral pathogens, in many different ways. Those microbiome changes may also impact indirect factors relating to corals, such as trace element availability (by microorganism uptake and thus less remaining for corals), and the feeding of corals on those microorganisms.

As an example, this linked article discusses the microbiome effect of nutrients in the context coastal runoff, and suggests that phosphate is a bigger factor than nitrate with respect to one particular coral pathogen expanding in numbers.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8902694/

There are several implications of this microbiome effect of nutrients to the topic at hand.

  • The effects of nutrient levels on coral health may vary tank to tank based on what microorganisms are naturally present.
  • Some of these effects (such as trace element bioavailability) may be “corrected” by dosing needed trace elements or sufficient particulate feeding (such as phytoplankton).
6. Finally, and very importantly, nitrate and phosphate are far from the only sources of N and P in a reef tank. They may not even be the most used sources. Other sources of N include ammonia (preferred by most corals studied over nitrate), urea (an organic excreted by fish), amino acids, other soluble organics, and particulates including detritus and whole phytoplankton, zooplankton, bacteria and other organisms. Likewise, there are many forms of phosphate available, mostly including dissolved and particulate organics and microorganisms. In fact, the reduced availability of particulate foods in reef tanks is a likely reason that early efforts to mimic ocean levels of nitrate and phosphate in reef tanks led to poor outcomes. We just were not allowing enough N and P availability.

The implications that aquarists need to be critically aware of are:
  • Nitrate level is a poor indicator of total N bioavailability.
  • Phosphate may also not be a good indicator of total P availability
  • Claims that, say, 0.5 ppm nitrate is plenty may not extrapolate to any aquarium other than the original one observed for reasons in 1. Same for phosphate.
  • Likewise, claims that 10 ppm nitrate is much better than 2 ppm are also difficult to generalize across aquaria for these reasons.
  • In general, having a few ppm nitrate may not be needed, or used by corals at all in some systems, but it is insurance that there is sufficient N.
How to Use This Info

I could drone on and on about these topics, and the discussion would get more and more detached from ordinary reefers experiences, so let’s focus on how one might use the info laid out so far, by looking at some very specific situations.

1. Limiting algae growth by limiting nutrients certainly works, but unless done in special circumstances, it usually risks coral problems from them not getting enough nutrients, or dino problems from them thriving at lower levels than other competitors that you might rather have.

The scenarios where this works are those where all photosynthetic organisms you want are getting enough particulate foods to supply N and P. Most algae types cannot consume particulates, so one can get some corals to thrive in low nitrate/low phosphate situations where algae cannot thrive. The Zeovit method does this, by providing various types of organics (bacteria, perhaps other materials). Not all corals can readily consume particulates foods, so this way to deal with algae is challenging and often fails. I personally think herbivores are much more likely to achieve success.

2. If you are in a high nitrate or high phosphate scenario, and are wondering if lowering levels is needed, take a step back before acting. Polls I’ve run suggest more problems come from lowering these levels than come from the levels themselves. Some fabulous tanks have had phosphate above 1 ppm and nitrate above 100 ppm. That doesn’t mean it’s always a good idea, but it does mean that such levels may not be the cause of the one or two corals you have that show poor polyp extension. Consider other factors as well. If you do proceed to lower them, which is a fine idea, proceed very slowly. Give corals time to adapt their transport capabilities. Plan to do it over weeks, not hours or days for a large drop.

3. Someone who claims a low level of nitrate works great should not be taken as evidence that you can lower it to that level and still be great. They may have far more of other N sources, such as ammonia, flowing through their system. There’s no simple way to test this, but folks who claim heaving feeding or who have lots of fish will necessarily have much more available ammonia than someone who feeds much less. Their corals may just be taking up the ammonia before it becomes nitrate. The same can be true of low phosphate and foods like phytoplankton or bacteria supplying P (and N) to corals.

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.
I don’t expect this article to reduce the debates about optimal nutrient levels in reef tanks, but I hope it can at least help reefers understand why there is such a debate, and potentially how to wade through it to get suggestions for their own systems.

Happy Reefing!
 
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bakbay

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How timely - I'm actually going thru this across my 3 tanks and it's rather confusing & frustrating at the same time. I didn't observe any particular pattern; except that zoas and other softies didn't like my 90gal's PO4 level. Guess the only parameter I can think of is "age of the tank"? You'll need to reach a level of "maturity" and I'm not sure if there's an indicator one can test for. I thought it was the growth of coralline algae but that has not been the case for me. Sorry about the rant - not very scientific, I know!

90gal: 1.5 year-old rocks, 6mos system, NO3: 150+, PO4: 0.2 - no WC. can't grow SPS but inverts, softies, and LPS are alive. Zoas didn't open until I reduced PO4 from 1.14 to 0.4. I'm not seeing growth yet so suspect that high NO3 is inhibiting growth?
150gal: 4 years old, very high nutrient (NO3: 150+, PO4: 1.5) - no WC. SPS, LPS, and softies are thriving!
450gal: 5 mos, NO3: 40, PO4: 0.9 - no WC. Can grow all corals except SPS. However, growth is minimal/non-existent.
 

Pod_01

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This bit is interesting, I just didn’t understand the why…

“This result also hints at why it can be very undesirable to lower phosphate too quickly: corals may suffer while they try to ramp up their capability to get what they need from the new, lower level.”

Also explains why some reefer run at low levels and corals are ok, corals are used to it.

But at the same time some reefers commented that they never observed this. They mentioned that they go down in PO4 and nothing happens. But as mentioned perhaps the tank has other sources of P…

Thank you,
 
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Randy Holmes-Farley

Randy Holmes-Farley

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This bit is interesting, I just didn’t understand the why…

“This result also hints at why it can be very undesirable to lower phosphate too quickly: corals may suffer while they try to ramp up their capability to get what they need from the new, lower level.”

Also explains why some reefer run at low levels and corals are ok, corals are used to it.

But at the same time some reefers commented that they never observed this. They mentioned that they go down in PO4 and nothing happens. But as mentioned perhaps the tank has other sources of P…

Thank you,

Maybe the corals were different, the drop wasn't as fast, corals were already limited in growth by something else (say, a trace element, light, etc). Could be lots of reasons.

The long awaited reef aquarium equivalent of the "Theory of Everything" remains equally elusive.

 

Pod_01

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It was discussion about Montipora, if I recall. From my observation they seem to show PO4 drop first, I suspect it is because of the fast growth when happy.

But I do like this point
corals were already limited in growth by something else (say, a trace element, light, etc).

This may contribute to lot of different observations and recommendations we all make. There are lot of potential things that can limit growth…
 

dvgyfresh

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I love this! I read somewhere that phosphate can encourage growth of corals but can be detrimental if the calcium /skeleton growth is not matching the speed ( IR high nutrient tank with low calcium/alkalinity could be a problem)
 

danlechem

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Great article Randy. Food for thought while my po4 is floating around 1!
 

wickedxreef

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Fantastic article Randy! Really loving these articles that you’re putting together. Keep them coming please.
 

SunnyX

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Great article! I have no idea what my nutrient levels currently are. Last year, I tested Phosphate and it was 0.90. I shrugged it off and went about my day.

Many people focus on chasing numbers while neglecting fundamental aspects like flow, water changes and a good source of live rock.

Nutrient levels vary so drastically between systems that I generally don’t find it necessary to know. One recurring issue I’ve noticed, and you’ve noted, is the problems caused by attempting to lower nutrient levels. This essentially opens Pandora’s box, unleashing various disruptions on your reef.

Keep it simple and don’t obsess over the numbers.

-Sonny
 
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Randy Holmes-Farley

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90gal: 1.5 year-old rocks, 6mos system, NO3: 150+, PO4: 0.2 - no WC. can't grow SPS but inverts, softies, and LPS are alive. Zoas didn't open until I reduced PO4 from 1.14 to 0.4. I'm not seeing growth yet so suspect that high NO3 is inhibiting growth?
150gal: 4 years old, very high nutrient (NO3: 150+, PO4: 1.5) - no WC. SPS, LPS, and softies are thriving!
450gal: 5 mos, NO3: 40, PO4: 0.9 - no WC. Can grow all corals except SPS. However, growth is minimal/non-existent.

What does 150+ ppm nitrate mean?

Does it mean around 150 ppm or a bit more, or does your kit top out there and it might be 300 ppm?
 

bakbay

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What does 150+ ppm nitrate mean?

Does it mean around 150 ppm or a bit more, or does your kit top out there and it might be 300 ppm?
I’ve diluted by 50% and got 75 (max on a Hanna HR) so it could be 150 or higher. I will need to dilute further to get better number.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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I’ve diluted by 50% and got 75 (max on a Hanna HR) so it could be 150 or higher. I will need to dilute further to get better number.

OK, I'd try 25% tank water and 75% new salt water, or maybe even 10% tank water. . :)
 

GARRIGA

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Reefs are devoid of measurable nutrients yet life goes on and I'm guessing the constant feed cycle and poop being introduced might be the cause of success or might not. Only one way to find out...
 

bakbay

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Great article! I have no idea what my nutrient levels currently are. Last year, I tested Phosphate and it was 0.90. I shrugged it off and went about my day.

-Sonny
What I've observed on my frag tank with mostly anemones, softies, and LPS (hammers): PO4 was 1.14ppm and the zoas didn't open and melted over time. Without changing anything, I've dosed PhosphateRx every other day and brought PO4 from 1.14ppm to 0.46ppm. Zoas started to open up at around 0.46ppm and I'm still dosing to bring it under 0.10ppm. However, NO3 was/is still off the charts at 150+ (will dilute more tonight and re-test).

Perhaps I can conclude that PO4 is more important than NO3? Anyway, it's a wild guess but I've observed that my tank is much "happier" now with PO4 not at elevated levels.
 

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