Nutrients keep climbing despite carbon dosing

[Scope]

Hey all,

I've posted a couple of weeks ago about my nutrients continually climbing. I thought I won't do anything crazy with it so I just started dosing aquaforest's bacteria and carbon source to feed heterotrophic bacteria and help bring nutrients down naturally. I'm currently at the highest recommend dose and the nutrients keep climbing regardless.

My tank is 500 liters total, I have a roller mat, octo regal 200 int skimmer and do 15-20% water change every 2 weeks. Here's a record of my parameters from the last 3 months. The tank is obviously young at barely 5 months but the corals aren't happy, I lost almost all SPS that were already growing and I'm at my wits end.

I have the following fish:

Hippo Tang
2x ctenocheatus tangs
Foxface
2 clownfish
Longnose butterfly
And like 3 other small fish (think chromis or goby)

I feel a pinch or 2 dry food and up to 4 cubes frozen, plus some nori.

 

[X-37B]

I use Vodka to keep no3 around 5
Po4 runs .1-.3.
Vodka.works.best for me after trying it all over 30+.
Started when no3 got to 25 I got up to 23ml a day before it started comming down.
I had to adjust down to keep it from bottoming out.
I now dose 5ml a day in my 150 to keep it at 5.
I adjust po4 when it hits .3 and bring it down to .1.
I do 5% a month WC.
I keep my dkh at 7-7.5.
I feed heavy with 10 fish in the system.

 

[EnterName]

I have two potential explanations:

1. The Easy one:
You are carbon dosing, and bacteria are assimilating nutrients, but at some point their population reaches a plateau, so there is no way for the bacteria to hold more nutrients and additional nitrogen has to leave via denitrification. Both run in parallel, but denitrification takes time, and it's possible your nitrogen import is faster than the actual export. Overall it's a pretty complex topic how effective actual denitrification in reef tanks really is.
Providing more surface area (more rocks or bio media) might be a potential way to speed up the process, but I don't know how well this scales, so don't expect a large effect.

2. The a bit more complicated one:
Carbon dosing can only work as long as carbon is the limiting factor for bacteria growth. Once something else becomes the limiting factor, adding more carbon won't help much because something else is missing for growth.
When we're talking about carbon uptake, iron (Fe) is one of the trace elements that can become a limiting factor very fast (Manck et al. 2024). So it's possible the supplied carbon can't be processed fast enough because the Fe-containing enzymes of central carbon metabolism aren't available.

Once we get to actual nitrogen assimilation, nitrification, and denitrification, a bunch of other trace elements become important. Nickel, for example, is used in urease, which converts urea (an organic nitrogen source) into ammonia so it can be assimilated by different organisms. Molybdenum-based enzymes reduce nitrate to nitrite, the first step of both denitrification and nitrate assimilation, so molybdenum is another candidate.

Copper is used for nitrous oxide reductase and nitrite reductases, so it might also play a role in the nitrogen cycle depending on what exact enzymes are being used. Note that copper is extremely toxic in marine environments, so I'd be very careful when supplementing it. Preferably only when an ICP-MS test suggests it.

In short: Trace elements might be missing. Iron is the most likely candidate. It can even become a limiting factor in nature, and when kept at natural levels you would need to dose it very frequently to keep it from bottoming out.

 

[Scope]

I have two potential explanations:

1. The Easy one:
You are carbon dosing, and bacteria are assimilating nutrients, but at some point their population reaches a plateau, so there is no way for the bacteria to hold more nutrients and additional nitrogen has to leave via denitrification. Both run in parallel, but denitrification takes time, and it's possible your nitrogen import is faster than the actual export. Overall it's a pretty complex topic how effective actual denitrification in reef tanks really is.
Providing more surface area (more rocks or bio media) might be a potential way to speed up the process, but I don't know how well this scales, so don't expect a large effect.

2. The a bit more complicated one:
Carbon dosing can only work as long as carbon is the limiting factor for bacteria growth. Once something else becomes the limiting factor, adding more carbon won't help much because something else is missing for growth.
When we're talking about carbon uptake, iron (Fe) is one of the trace elements that can become a limiting factor very fast (Manck et al. 2024). So it's possible the supplied carbon can't be processed fast enough because the Fe-containing enzymes of central carbon metabolism aren't available.

Once we get to actual nitrogen assimilation, nitrification, and denitrification, a bunch of other trace elements become important. Nickel, for example, is used in urease, which converts urea (an organic nitrogen source) into ammonia so it can be assimilated by different organisms. Molybdenum-based enzymes reduce nitrate to nitrite, the first step of both denitrification and nitrate assimilation, so molybdenum is another candidate.

Copper is used for nitrous oxide reductase and nitrite reductases, so it might also play a role in the nitrogen cycle depending on what exact enzymes are being used. Note that copper is extremely toxic in marine environments, so I'd be very careful when supplementing it. Preferably only when an ICP-MS test suggests it.

In short: Trace elements might be missing. Iron is the most likely candidate. It can even become a limiting factor in nature, and when kept at natural levels you would need to dose it very frequently to keep it from bottoming out.

Both ICP tests from a few weeks ago have shown that my iron levels are rather low. So it could be a possibility. I should probably send a new ICP sample and dose based on the result.

 

[EnterName]

Both ICP tests from a few weeks ago have shown that my iron levels are rather low. So it could be a possibility. I should probably send a new ICP sample and dose based on the result.

For Iron it will be hard to dose based on the results. I can almost guarantee you that you will see low levels even if you dosed a day or two before. Some people keep iron levels several orders of magnitude above natural levels for that very reason.

Trace element solutions to dose daily like "ATI Daily Traces #A" and "Tropic Marin Pro-Coral K+ Elements" include iron as well. These may be the right choice here.

 

[Scope]

For Iron it will be hard to dose based on the results. I can almost guarantee you that you will see low levels even if you dosed a day or two before. Some people keep iron levels several orders of magnitude above natural levels for that very reason.

Trace element solutions to dose daily like "ATI Daily Traces #A" and "Tropic Marin Pro-Coral K+ Elements" include iron as well. These may be the right choice here.

I think I'll dose it for some time and then see if the nutrients start to go down. Thanks for pointing me in (hopefully) the right direction.

 

[Randy Holmes-Farley Verified Community Expert]

My target nitrate level is 5-50 ppm, so I’m not sure I’d focus on lowering it. I’m not sure exactly what the AF product is or how they determined a max dose. Many people find carbon dosing lowers nitrate, but generally slowly, and phosphate not much.

Some great tanks have 1 ppm phosphate:

https://www.reef2reef.com/ams/randy’s-thoughts-on-nutrient-target-ranges.1087/

 

[EnterName]

I think I'll dose it for some time and then see if the nutrients start to go down. Thanks for pointing me in (hopefully) the right direction.

GFO for phosphate might actually be much more useful than carbon dosing by the way. The phosphate bound to rocks will replenish dissolved orthophosphate whenever you perform water changes. It takes a while until those phosphate depots are depleted so binding it to something else works faster. However, I would get rid of a bit of nitrate first, so carbon dosing makes sense, but if you keep adding carbon and then suddenly lift a limitation you might end up with a bacteria bloom or some really weird growth. I once had some brown slime clog the tubing to my UV sterilizer... It was really nasty.

Edit: As Randy pointed out you are within his target ranges so it might be worth considering if you need to lower them. I personally like to keep my nutrient levels far lower as I have had the best results with that and it makes acclimating corals easier in my opinion. The sudden phosphate spike new corals might have to deal with when being introduced into your tank is (in my opinion) a valid reason to try to lower nutrient levels, but obviously Randy is right in saying that there are very good and healthy looking tanks with those levels.