Relationship between alk, NO3 and PO4 and how this relates to corals and zooxanthella

Tripod1404

Active member
I have been reading bunch of thread about nitrate, phosphate and alk. I decided to write something up based on how these parameters are related and how they are related to the corals and to the zooxanthellae.

I think through the years it became somewhat of a "common knowledge" that there are two main approaches to a sucsefull coral tank. First it to have "high nitrate, high phosphate and high alk" and the second is to have "low nitrate, low phosphate and low alk".

One important thing to note here is keeping two of these low and one of them high is stressful for corals. When zooxanthellae (zoox) have access to high amounts of, say nitrate but low amounts of phosphate, they starve the coral of phosphate by taking up all the phosphate that enters to the coral cells. This cause coral cells starve and they either expel zooxanthellae, bleach, starve and die, or they die because of phosphate starvation before they can expel the zoox. First option causes STN, second option causes RTN. Keep in mind there are a lot more stuff that can cause STN or RTN, some of which are pathogenic, this is just one of the potential causes.

Same thing happens in reverse direction when nitrate is low and phosphate is high.

And this all connects to alk based on the demand of corals to zooxanthellae, as well as N and P.

If you have high nitrate and phosphate, you should also have high alk. This way corals can built their skeleton faster. When they built skeleton faster, they can use higher amount of N and P, and the carbon provided by zoox to built soft tissue faster. Newly made corals cells also get some of the zoox as well, so coral cells do not get over run by zooxs.

If you have high nitrate and phosphate but low alk. Rate at which corals grow gets limited by alk. This causes a surplus of N, P and the carbon fixed by zoox simply because corals do not have enough new skeleton to use these for building more soft tissue. All this surpass N and P fuel zoox and causes the intercellular zoox numbers to increase and coral browns. Corals can live browned, but in extreme cases corals will bleach because number of intercellular zoox gets to high that they start to starve the coral cells. So they expel zoox and bleach. This is the case of "my acros were brown but they started to bleach"

If you have low nitrate and/or phosphate but high alk. High alk inevitably cause corals to built skeletons faster but this time corals do not have enough N and P to built soft tissue. As a result, coral cell and the intercellular zoox need to compete for the same limited amount of N and P. This means one of the two would eventually starve. If coral cells starve, they expell zoox and bleach. This most easily happens at the tips (or places that get the most light) since those are the parts of the coral that is growing the most, and so has the highest demand for N and P. That is why increasing alk too fast causes burned tips, it is simply this event happening locally. In extreme cases a coral can completely burn, or in other words, bleach. On the other hand, If zoox starve, corals get pale as they are forced to live with a very low number of zoox. N and P cannot simply support more zoox. If you do this moderately, you get a zeovit tank. In such a tank, there is so little zoox in corals that you simply see the fluorescent proteins of coral against their white skeletons, this is what that makes them very brightly colored. I believe they also produce more fluorescent proteins to protect what little number of zoox they have.

SPS corals are particularly sensitive to N, P and alk because amount of soft tissue they can have is limited by amount of skeleton. LPS can, to some extend, have more soft tissue with same amount of skeleton. A hammer for instance, can have a bigger polyp with more tentacles while keeping the its corallite about the same size. Softies dont even need skeleton, so they are far less sensetive to interplay of alk N and P.

Long story sort, in either case, you should not limit the growth of corals. If one of the N, P or alk become limiting (keep in mind if one is excess it means other became limited), it causes coral and zoox to start competing for the limitted resources. When this happens, there is no point of having a symbiotic relationship. If zoox competing with the coral cells for resources, it is no longer a symbiont but a parasite. This results in coral cells to expel zoox and bleach.
 
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Nice write up.

What do you consider high and low for N, P and alk?

I think high & low should be defined relative to each other.

Obviously NSW levels for all represent the "lowest" , which is; nitrate and phosphate barely detectable with hobbyist grade kits while alkalinity is around 6.7-7 dKH.

But say, if your nitrate is around 5ppm, phosphate should not be low and alkalinity should not be low. So say, for 5ppm nitrate, 0.06 ppm phosphate and alk around 8.5-9 dKH.

People achieve success with much higher N, P and alk levels, so if you increase one you should also increase others. Or if you want to keep one low, other should also be low.
 
Ah ha!!
Alk 9.6 (slowly coming down from almost 11).
NO3 0.5
PO4 undetectable (all Salifert).

Lighting good enough. Flow great.

Can't keep SPS alive for more than a month or two. Acans struggling. Other LPS and softies doing great. You've given me something very interesting to try.

I ballanced Alk and Ca and now I'm going to let both fall naturally to Alk of 8.0 and try to keep PO4 and NO3 where they are. Will test Alk daily to catch it when it reaches 8. Feeding Reef Energy and Reef Chilly a couple times a week to hopefully avoid starvation and cushion the change.

"Only bad things happen fast in a reef tank."

Thanks for the write up! I'll let you know if it helps.



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Ah ha!!
Alk 9.6 (slowly coming down from almost 11).
NO3 0.5
PO4 undetectable (all Salifert).

Lighting good enough. Flow great.

Can't keep SPS alive for more than a month or two. Acans struggling. Other LPS and softies doing great. You've given me something very interesting to try.

I ballanced Alk and Ca and now I'm going to let both fall naturally to Alk of 8.0 and try to keep PO4 and NO3 where they are. Will test Alk daily to catch it when it reaches 8. Feeding Reef Energy and Reef Chilly a couple times a week to hopefully avoid starvation and cushion the change.

"Only bad things happen fast in a reef tank."

Thanks for the write up! I'll let you know if it helps.



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If you have one, I would remove GFO as well. Imo gfo should only be used in tanks that have phosphate leaching rocks or massive algae problems (two of which are actually related). Overuse of GFO causes issues because it drops phosphate levels to very low amounts but nitrate doesn't drop that much. It causes the high N low P situation I explained.
 
I have found that I can run higher intensity lights with higher levels of nutrients.

That is correct. It also comes down to the utilization of nutrients and alk.

If you have high nutrients, giving them more light allows corals to grow faster and utilize N and P.

If you have high light but not enough nutrients, it increase the demand of zoox on N and P . This, in extreme cases, cause corals to bleach (or turn very pale), since corals expel some or all of zoox to reduce the demand in N and P.
 
Thanks for the write-up, very interesting.

I have the same question as salty joe - what do you consider low?

I have a 6 month old tank with a few leathers and other soft corals, which I hope to build up in future. Currently my Alk is 9 and Phosphate is 0.25 ppm. I run Marine Pure and so far it is keeping Nitrate at 0. Phosphate and Nitrate tested by API kits, Alk by Red Sea. Dosing Aquaforest 3 part and using their salt. Ca is 470 and Mg 1480 currently. I have some GHA but it seems to be dropping and have started changing water to try and drop the Phosphate over time to help keep the GHA under control.

What do you think about keeping Nitrates 0, aiming to reduce Phosphate but keeping Alk at 9?

Thanks!
 
Thanks for the write-up, very interesting.

I have the same question as salty joe - what do you consider low?

I have a 6 month old tank with a few leathers and other soft corals, which I hope to build up in future. Currently my Alk is 9 and Phosphate is 0.25 ppm. I run Marine Pure and so far it is keeping Nitrate at 0. Phosphate and Nitrate tested by API kits, Alk by Red Sea. Dosing Aquaforest 3 part and using their salt. Ca is 470 and Mg 1480 currently. I have some GHA but it seems to be dropping and have started changing water to try and drop the Phosphate over time to help keep the GHA under control.

What do you think about keeping Nitrates 0, aiming to reduce Phosphate but keeping Alk at 9?

Thanks!

I think high & low should be defined relative to each other. It is impossible to find a magic number that would work for everyone. This is why I am trying to avoid giving numbers for what is low and what is high. There are people who keep successful SPS tanks with NSW levels of alk and nutrients(~0 ppm nitrate, 0ppm phosphate and alk around 7 dKH. And there are people who keep tank with a lot more nutrients (like 50ppm nitrate, 0,2 ppm phosphate and alk around 10dKH) Check this article below for the high end. So for someone in the middle of these two can be considered both high and low depending on from whcih direction you are looking at. I think the best way is to observe corals and not making changes rapidly. If you want strip water of any single nutrient, you need do it for both and reduce alk. If you want to have one high, you need to keep both and alk high. Main idea is not having 0 of one parameter and non-zero for the other one.

In your case, phosphate of 0.25ppm is considerably high (can it be 0.025?). In that case, I would keep nitrate at 5 ppm. Alk at 9dKH is okay, you might even increase it a bit more once you have stony corals. Softies are very forgiving when it comes to balance of alk and nutrients.


https://reefs.com/magazine/skeptical-reefkeeping-ix-130/
 
I think high & low should be defined relative to each other. It is impossible to find a magic number that would work for everyone. This is why I am trying to avoid giving numbers for what is low and what is high. There are people who keep successful SPS tanks with NSW levels of alk and nutrients(~0 ppm nitrate, 0ppm phosphate and alk around 7 dKH. And there are people who keep tank with a lot more nutrients (like 50ppm nitrate, 0,2 ppm phosphate and alk around 10dKH) Check this article below for the high end. So for someone in the middle of these two can be considered both high and low depending on from whcih direction you are looking at. I think the best way is to observe corals and not making changes rapidly. If you want strip water of any single nutrient, you need do it for both and reduce alk. If you want to have one high, you need to keep both and alk high. Main idea is not having 0 of one parameter and non-zero for the other one.

In your case, phosphate of 0.25ppm is considerably high (can it be 0.025?). In that case, I would keep nitrate at 5 ppm. Alk at 9dKH is okay, you might even increase it a bit more once you have stony corals. Softies are very forgiving when it comes to balance of alk and nutrients.


https://reefs.com/magazine/skeptical-reefkeeping-ix-130/



Thanks for the feedback and the links, interesting reading.

I am currently using an API test kit and it's lowest reading is 0.25 ppm. Mine is > 0 and < 0.25 ppm but I can't get more accurate than that. Obviously I need a better test kit so I will look into the best one. I use the Red Sea kits for my Alk, Ca and Mg so will check that out.

I understand the point about not chasing numbers. For the moment I think I will focus on gradually reducing Phosphate through water changes and if it doesn't budge enough consider other options. I don't really want high Phosphate so will focus on reducing that rather than increasing Nitrate. I do have a small algae problem so this seems my best approach for the moment.

I'm not doing anything to control Alk apart from using the Aquaforest salt and dosing solutions, and it is stable, so will just continue that and monitor to see what happens. One thing I notice compared to what I am reading in the forum sis that Aquaforest seems to favour/produce high Mg, so I am currently using that as my main high water mark (I.e. keeping daily dosing levels at a point where Mg doesn't climb any higher than it's current ~1500 mark). I was targeting Alk in the 8-10 range which is where it is now. I figure with my current level of experience I am best doing what Aquaforest suggest for now and dosing all the 3 components equally.

Once I start adding stony corals I guess things may change so I'll just keep an eye on all the parameters. The tank is still young also and again will evolve over time.

Thanks again for all the info, it's helping me build a better understanding of my tank.

Regards, Brad.


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@Tripod1404: Great write up and read. Goes a long way to explaining what's been happening in my tank and where my past successes and failures have come from. Thanks!


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One important thing to note here is keeping two of these low and one of them high is stressful for corals. When zooxanthellae (zoox) have access to high amounts of, say nitrate but low amounts of phosphate, they starve the coral of phosphate by taking up all the phosphate that enters to the coral cells. This cause coral cells starve and they either expel zooxanthellae, bleach, starve and die, or they die because of phosphate starvation before they can expel the zoox. First option causes STN, second option causes RTN. Keep in mind there are a lot more stuff that can cause STN or RTN, some of which are pathogenic, this is just one of the potential causes.


I just want to say that this explanation is the clearest I've ever read about how to keep SPS and understand RTN / STN. Thank you for providing it.
 
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