Dissolved Organic Compounds

[RickyV]

I am often asked why I do big and frequent water changes in a planted tank if the plants are consuming the excess nutrients. From what I have read on several forum posts is that there is a correlation between water changes and reduced algae and increased plant health. The most common reason said to explain this correlation is that water changes remove dissolved organic compounds.

This is my interpretation of the forum posts I have read so please correct me if I am wrong. Dissolved organic compounds include things like proteins, lipids, carbohydrates that algae can easily utilize as nutrient sources. Plants cannot utilize these dissolved organic compounds as easily (or at all?), and need them to be converted to inorganic nutrients like nitrates and phosphates to use them. Algae does not need to wait for them to be decomposed into these inorganic nutrients so they thrive with high levels of dissolved organic compounds.

I have also read that dissolved organic compounds are removed because they significantly increase the levels of aerobic bacteria which can reduce the oxygen levels in an aquarium. However is this really a concern with enough gas exchange in the tank? Or is it possible for the bacteria population to get so high that it would take a very unreasonable amount of gas exchange to supply enough oxygen to keep DO at optimal levels?

This gets me to my next question, if bacteria are consuming DOCs then they are competing with algae, which would be good? So would another method to assist in the removal of DOCs and thus reduction in algae be to use very large wet dry filters? Then plants utilize what is generated from the wet dry filter. I actually don't use any bio media in my planted tanks, however this is making me rethink that.

 

[ElleDee]

This is a good question, and not one I have a great answer to, but I can share my (sparse) notes.

I suspect the situation is quite complicated because DOC (or DOM, if we want to include the carbon and non-carbon fraction) is covers a diverse set of compounds and they don't all have the same effect in a planted aquarium. I think it's important to include larger organic waste too, not just the dissolved fraction, because that is being removed in water changes as well.

I do think you've pointed to some of the issues, such as it causing a proliferation in unwanted bacteria and a resulting drop in DO. As I understand it, easily decomposed matter needs so much oxygen that it's the rate limiting factor - that adding more just speeds up the process rather than adding more oxygen to the system for the other organisms. I have read that there are components of DOC that some algae can use directly (maybe some amino acids? I don't remember), but this was a paper on algae blooms and not all species had the same ability. There's probably more scholarship that's already been done here, but I haven't done a deep dive.

I feel like they are also related to REDOX/ORP values which, umm... I don't really understand! Seems like it could be important, but my brain fogged over the last time I read about it.

DOCs are not all bad though. I only do low tech tanks, and decomposing organic matter is a primary source of CO2. I am wary of having too much of a good thing, but I definitely need to have some amount for the carbon.

Humic substances all count as DOC, and those have health benefits to fish, act as chelators and can increase nutrient availability to plants, and are a critical component of blackwater systems. I have also read some studies that they have algaecidal properties, though I don't know if that can be meaningfully implemented in the hobby without tinting the water.

I would love to hear a more comprehensive overview on the topic and not just the same little factoids passed along without elaboration.

 

[Zeus.]

Our tanks are relatively closed ecosystems of small volume with relatively small buffering capacity. The marine side of the hobby use protein skimmers to help buffer the water ( Not 100% needed as my son has gone marine and doesn't have a protein skimmer and runs it on a Fluval FX6, one year on plus and its looking great IMO)

So a WC (Water Change) is a simple cost effective way of reducing DOC (and excess ions and cations) and polishing the water, resetting the tank and negating the need for testing tank parameters.

I actually don't use any bio media in my planted tanks

And why should you, the surface area off all the roots in the media far exceeds the surface area of any bio media in your sump or canister. Fish only tank need Bio Media as there's nowhere else for the needed bacteria to form their ecosystem.

Plus bio media reduces the output of filters reducing the flow/turnover in the tank. Flow/turnover is KING of the CO2 injected tank.

 

[Art]

I would love to hear a more comprehensive overview on the topic and not just the same little factoids passed along without elaboration.

@RickyV Thank you for posting this very interesting question that I do think needs more thought, research and conversation. @ElleDee I think you nailed what is needed.

In all my years in this hobby, I have not been able to determine the why behind some of the practices we believe are necessary for a successful (little algae / plants healthy) planted aquarium. The water change to reduce dissolved organic compounds is one good example. It is what we do but we can't explain why reducing or removing DOCs results in reduced algae in the planted aquarium.

Consider this, in the past, I have tested no water changes and instead using large pore carbon to remove DOCs. Theoretically, forcing the water through the carbon properly should result in near 100% removal of DOC from the water. A 50% water change would only reduce DOC by 50%. Hypothesis was that, as a result, carbon alone should have the same or better effect than large water changes. Wouldn't the hobby be so much better if we didn't have to do large water changes?

Unfortunately, the hypothesis didn't hold true based on my observations. The tank did better with water changes than without and only using carbon. Why would that be???

Water changes must be doing something more than just removing DOC. Of course, this is assuming that we are correct in our assumption that removing DOC is bad for algae and good for plants. We haven't addressed the question, why can algae presumably make use of DOC while aquatic plants can't?

Given how small our hobby is, we may never have the intensive research needed to answer these questions. I'm hoping at some point water treatment research reveals why removing DOC will eliminate algae versus higher plant life. I don't have high hopes, though.

What I've resigned myself to is "why ask why if what you are doing is working?"

 

[Pepere]

In all my years in this hobby, I have not been able to determine the why behind some of the practices we believe are necessary for a successful (little algae / plants healthy) planted aquarium. The water change to reduce dissolved organic compounds is one good example. It is what we do but we can't explain why reducing or removing DOCs results in reduced algae in the planted aquarium.

Well, sometimes, just knowing what works is good enough…

To be honest I cant explain at the molecular level why eating satisfies my hunger, or sleep helps fatigue, or why it feels better up next to the woodstove on a cold day, but it is enough to me to know what works….

To me 50% weekly water changes are so much easier than constantly fighting algae with a water top off only regimen…. Making water changes easier seems a much better effort than avoiding them at all cost…

 

[ElleDee]

@RickyV
Unfortunately, the hypothesis didn't hold true based on my observations. The tank did better with water changes than without and only using carbon. Why would that be???

Can you elaborate on what you did exactly the changes you saw? I've never used carbon, and I've heard very mixed opinions.

 

[Art]

To be honest I cant explain at the molecular level why eating satisfies my hunger, or sleep helps fatigue, or why it feels better up next to the woodstove on a cold day, but it is enough to me to know what works….

Well said. I’ve accepted this but my analytical brain objects sometimes.

Can you elaborate on what you did exactly the changes you saw? I've never used carbon, and I've heard very mixed opinions.

Sure. So my hypothesis was, if DOC is bad and a 50% water change only removes 50% of it, then removing all of it via carbon would yield better results.

I first wanted to 1) find the best carbon and 2) confirm whether it removed needed nutrients.

Confirming the best carbon took research but I settled on three- Purigen, small pore carbon and a hybrid large pore carbon. I did a color removal test to see which was quickest and best.

I first set up composters for each to force water through them. Here is Purigen and hybrid.



Then I set up a test aquarium for the two and determined how fast and completely they removed the coloring. You can see the test aquarium here:



Then ran same test with other carbon to determine that the hybrid large pore was the best.

Then I used chelated Fe to see if it would remove it from the water. I don’t take a picture of this because, well, it was just a bucket of water. Bottom line, the large pore didn’t remove any Fe from the water in the 24 hrs I ran the test.

Once I was comfortable with the carbon to use, I set up the canister in the aquarium and ran it 24/7 for a month. No water changes but everything else the same. I changed the carbon out every two weeks.

Conclusion: the water was MUCH clearer than usual but the plants were not at their best. Green algae was starting on the glass and some hair algae was visible.

Now, maybe the carbon was getting clogged and that caused it to strip out some nutrients. Or, the increased clarity caused more light to be available than usual. Or, decreased DOC caused bacteria to decrease due to less food resulting in some imbalance. Who knows.

I still believe carbon is a good thing to use correctly to reset DOC levels once in a while but in combination with continued water changes.

 

[Burr740]

@Art There was an article on the kleb (klib? whatever that site was called, barr used to cite it a lot) showing icp test results that it definitely had an impact on the secondary micros, dont remember what it said about about Fe

Like you and many others Ive also seen plant issues when Ive tried to use it long term. I do use it sometimes but never more than a few days, week or so

 

[Pepere]

Well said. I’ve accepted this but my analytical brain objects sometimes.

I get that too, and I applaud your trying things out…

George Farmer is fond of saying in regards to algae, prevention is easier than a cure…. Realizing this is true makes me loathe to reduce water change depth or frequency…

When I try an intensifcation of light intensity or duration, I make sure I spend a good 5 minutes carefully studying my tank and plants a few times a day for at least a week afterward…

Recently, I noticed my Samolous Parviflora growing ok, but the leaves were narrowing and losing red getting green. I added some root tabs, increased watercolumn ferts and notched up co2 and intensified lighting. Within 2 days my rocks were covered with unsightly algae wheras before they would go a week or two between being pulled out and scrubbed with a stiff brush and peroxide. I serviced them and by weeks end I decided to just pull them from the tank. The Samolous was responding well to increased intensity. I decided to sacrifice rocks for more plants…

I am thinking the reason for no hardscape in Dutch Aquascaping is to allow more plants and more lighting intensity…



Before.

After removing rocks.

New growth getting redder..

I have decided to work on increasing the numbers of some of the plant stems and then reducing total species to 7 as I can fill tank out with just 7 plant species and to try my hand at Dutch style with this tank.

My other display tank has rocks fairly well shaded by plants so they stay clean longer…

 

[Burr740]

I am thinking the reason for no hardscape in Dutch Aquascaping is to allow more plants and more lighting intensity

Nah, its because the goal of a Dutch is to create the entire structure and all its effects using plants alone, not rocks and wood

 

[Pepere]

Nah, its because the goal of a Dutch is to create the entire structure and all its effects using plants alone, not rocks and wood

I was being facetious, thinking someone started struggling with algae on their hardscape and decided, what the heck, let’s make a new aquascaping category with a no hardscape rule. Solves my problem and gives me more room for plants…

It may well have no basis In reality, but it sure is an attractive explanation in my mind. Never let the truth stand in the way of an amusing story….

 

[Art]

There was an article on the kleb (klib? whatever that site was called, barr used to cite it a lot) showing icp test results that it definitely had an impact on the secondary micros, dont remember what it said about about Fe

It's the Krib (Activated Carbon and Aquatic Nutrients - I). I read that and the author does admit that there may be differences as a result of the type of carbon used. The one he used is probably a micropore type versus the one I used that is macropore. The one with the larger pores is much less likely to capture the chelates associated with Fe and other micronutrients.

This squares with ADA that uses bamboo carbon as part of their line. Bamboo carbon is a macropore carbon and much less likely to materially impact nutrients.

Also, please note that I auto-dose on a daily basis so even if some was being pulled, more was going in daily.

All that to say that I'm just not sure. Safest bet is to continue to do your water changes and consider using carbon sporadically to do a deep cleaning of the water. However, you must use the correct carbon and use it actively (canister) versus passively (hanging in a bag). IMO.

 

[Koan]

Unfortunately, the hypothesis didn't hold true based on my observations. The tank did better with water changes than without and only using carbon. Why would that be??

Well and I wonder if we're not asking the wrong question here.

The assumption I think that you made with those filters is that pore size equals efficacy of extraction.. but do we know that different pore size has the desired effect on hydrophilic versus hydrophobic vs polar compounds? lipids in solution, aminos in solution phytoalkaloids in solution..?

What if the question isn't how do we get it out, but what was the problem with having it there in the first place?

I know that Clive was absolutely passionate that

1) the point of DOC was not anything to do with algae metabolism directly, but rather as Ricky specifically asked in the OP, the sheer magnitude of volume of oxygen that bacteria subtract from our tank water while they're feasting on all these nutrients. Subtract both in their process of living and reproducing, and also in the process of taking NH4 and turning it into NH3 and Co2 , hoovering up more oxygen in the process of that work of metabolism, as well as by jamming two more molecules of oxygen onto every carbon gas molecule every time.

This oxygen deficit then putting every plant, and our livestock, at significant and ongoing stress.

And 2) further that these proliferating bacteria then go to town creating a nasty biofilm coating the plants' transpiration surfaces, and further preventing them from absorbing what oxygen is left. And from absorbing dissolved Co2.

And this triple stressing of the plants causes them to degrade more leaves, eject more nutrients, provoke up more algae blooms.. and around we go.

I know Art's experiment tanks did not have plants but since our tanks all do I'm not 100% sure what that tells us.

I have not found a citation for specifically how severely oxygen is depleted by bacteria feasting on DOC and detritus and metabolizing ammonia and nitrate, but Clive at least was emphatic that BOD, or the Biological Oxygen Demand that results from build up of organic waste in tanks, is very much the point, and I get the impression this is something that is a well known topic in sewage treatment design papers, out there somewhere.

Certainly we have absolutely no way to measure dissolved oxygen in this Hobby, making it a great candidate for the key nutrient / stressor/ x factor that we're missing? The one thing our plants never experience out in the wild is oxygen depletion, they simply won't grow whether there's not enough oxygen. So it stands to reason that they are extremely poorly adapted to manage hypoxia at all.

 

[Art]

There used to be an old German aquarist that wrote two books about planted aquariums back in the 80s. He also wrote many magazine articles on the many issues associated with low oxygen levels in tanks. Very controversial at the time.

I don’t quite understand Clive’s point though. Let me see if I can summarize.

1. DOC in the aquarium will increase bacterial populations because the bacteria feed on it. This is proven and forms the basis for carbon dosing in the reef aquarium.
2. Increased autotrophic bacteria will increase oxygen uptake given greater population. It is also proven that PO4 and NO3 will decrease.
3. Increased bacterial colonies will also increase bacterial slime. This will coat many surfaces that may include plant leaves. You see this also with cyano.
4. Clive is suggesting that decreased oxygen levels negatively impact plants and fauna. This is an assumption. We do not see the usual gasping by fish or hanging out at the surface associated with low oxygen. Also, algae would also be negatively impacted if levels were so depleted. I don’t see this as likely especially since many aquariums do have artificial air supplementation that should at least reach oxygen equilibrium.
5. Bacterial film covering plant leaves preventing gas and nutrient exchange is also an assumption. Wouldn’t this also happen to algae? This bacterial film is relatively easy to see and feel. Pull out your bacterial filter and feel the media. All the plants would feel this way if this were true. Besides, most plants would be able to uptake nutrients and gas exchange via their roots, wouldn’t they?

I like thinking that there is more at work here other than simply DOC feeds algae and not plants. However, I am respectfully not convinced with his analysis.

 

[Koan]

We do not see the usual gasping by fish or hanging out at the surface associated with low oxygen

But this is exactly what we do see, whenever we do too deep a cleaning of the substrate. The cloud of gunk translocates extremely large bacterial populations immediately into the water column, where dissolved the oxygen levels are so much higher than in the detritus, allowing that very large new suspended population to exponentially multiply and eat up that oxygen.

In terms of persistent increasing oxygen deficits, animals have capacity to adapt that plants don't have. All animals can and do ramp up their erythrocyte / hemocyte count in reaction to long-term oxygen depletion.

Also, algae would also be negatively impacted if levels were so depleted.

Actually they would not be so affected: algae are by definition single celled organisms that absorb all nutrients directly across their cell membranes. Algae do not require any secondary transport to tissues distant from transpiration surfaces the way plants do, it does not cost algae any energy to move.

These are photosynthetic algae cells that are creating more oxygen than they need, and with minimal housekeeping burden during dark non-photosynthetic time. Compare this to plants' orders-of-magnitude higher tissue metabolic load to keep maintained during non-photosynthetic times.

artificial air supplementation that should at least reach oxygen equilibrium.

Okay but you're assuming that BOD in a tank with high bacterial food content does not outstrip the rate of dissolution of air from airstones HOB filters etc.
I think the point is we don't have grounds for that assumption, and no way to measure it I stand corrected:

"..The Winkler method involves filling a sample bottle completely with water (no air is left to bias the test). The dissolved oxygen is then "fixed" using a series of reagents that form an acid compound that is titrated.. Dissolved oxygen field kits using the Winkler method are relatively inexpensive, especially compared to a meter and probe..."​

​

Looks like Hanna sells the kits

 

[Art]

But this is exactly what we do see, whenever we do too deep a cleaning of the substrate. The cloud of gunk translocates extremely large bacterial populations immediately into the water column, where dissolved the oxygen levels are so much higher than in the detritus, allowing that very large new suspended population to exponentially multiply and eat up that oxygen.

If this happens, yes, I agree. I'm just saying that I don't see it in other circumstances. Many, many people do not vacuum the substrate when they do water changes. I certainly didn't for many years. For example, I never had issues with oxygen levels to the point where fish were gasping.

Actually they would not be so affected: algae are by definition single celled organisms that absorb all nutrients directly across their cell membranes. Algae do not require any secondary transport to tissues distant from transpiration surfaces the way plants do, it does not cost algae any energy to move.

Understood. However, if biofilm coats the algae, even with a simplistic transport mechanism, oxygen won't be able to get through to their cells. So, I don't see how the biofilm impacts plants but not algae.

Okay but you're assuming that BOD in a tank with high bacterial food content does not outstrip the rate of dissolution of air from airstones HOB filters etc.

Yes, I am making this assumption. It's based on my observation of tank inhabitants only.

I guess this is the point. A lot of what is being discussed is assumptions based on observations. Until someone decides this is worth a scientific inquiry and does the hard work, "the world may never know!" That's an old reference to a lollipop commercial for you young ones!

My personal feeling is that increases in DOC does increase bacterial populations that results in a significant decrease in NO3 and PO4 and may also impact some trace elements. This impacts the health of plants thus reducing their ability to take up nutrients and keep algae at bay. Weaker plants means single celled organisms like algae have the advantage so their population grows.

This creates a downward spiral in the tank's environment. This can certainly be exacerbated by biofilm coating plant surfaces. The increase in bacterial respiration will also certainly pull down oxygen levels. Again, this would exacerbate the problem.

All of the above said, it is just conjecture and my personal opinion.

So would another method to assist in the removal of DOCs and thus reduction in algae be to use very large wet dry filters?

Getting back to one of @RickyV's original questions, why not use macropore carbon that would reduce water-column DOC way more than a water change? This reduction in DOC would keep bacterial populations from going crazy which is the point of @Koan's fine points. Or, am I missing something?

 

[ElleDee]

I feel like the question is if the relationship between organic pollution and dissolved oxygen in our tanks (ie a system where we actively supplement oxygen in contrast to a stagnant eutrophic pond) is linear or not. My sense is that it's not, that every tank has a threshold based on the amount of oxygen in the system where it can handle x amount of waste and if you are below that then the downward spiral Clive describes isn't really in play. I am with @Art and also skeptical that hypoxia is an issue in most tanks with basic maintenance, even those that fall way short of the 50%-ish weekly target that many people swear by. I don't deny that the big water changes are beneficial, I just am not convinced in the reasoning given. I can be convinced, but I need better evidence because it doesn't really fit with my personal observations.

In terms of persistent increasing oxygen deficits, animals have capacity to adapt that plants don't have.

You have a citation on that? You are not the first person I have heard this from, so I don't mean to single you out. I'm genuinely curious where this idea is coming from. (And honestly the reason I didn't ask the first person I heard it from was because that person was unpleasant to interact with.)

But it just doesn't smell right to me. Plants have adaptive measures for everything and are capable of incredible plasticity in response to environmental challenges, why wouldn't they for low oxygen availability? Surely aquatic plants have some tricks up their sleeves to address this issue that they surely face in the environments they evolved in.

 

[Koan]

But it just doesn't smell right to me. Plants have adaptive measures

I think when I said capacity you heard ability, which is not what I intended to convey.

Animals have much more capacity, or if you like range / margin / ease of adaptation compared to plants. All a vertebrate animal has to do is speed up the precursor-maturation process to produce some more erythrocytes : minimal energy expenditure, minimal time, done. Invertebrates can grow more hemocytes, although their range of adaptation is narrowed compared to vertebrates.



By comparison everything a plant needs to do to adapt is expensive and slow, because by definition plant structural changes compared to animal structural changes are expensive and slow. It's what it means to be a plant.

Plant macrostructures are fixed within their phenotypic parameters. They don't have reservoirs of erythrocyte precursors waiting in the wings. They can produce their immersed form. They have some range of structural variation, produce some more aerial roots etc. But subjected to progressive relative hypoxia in that environment, insufficient relative to the requirements that plant evolved to utilize, they cannot then construct more lacunal density to transport more oxygen.

They have no way to speed the oxygen transport process to starving tissues. Plants can only adapt to excessive hypoxia at the intracellular level: they re-engineer their mitochondria, they transition to alternate, less efficient ATP generation pathways and amino acid metabolic pathways, etc. Reengineering and transition processes that are energy expensive and slow.

Expensive and slow in a nutrient limited environment like underwater, equals stress.

 

[Art]

So that hurts my brain but thanks for posting. At a high level, it does make sense to me.

 

[Koan]

So, I don't see how the biofilm impacts plants but not algae

This is a great question! One thing is that when looking at big algae structures like threads and staghorns it's easy to forget that algae don't have leaves, they don't have fixed transpiration surfaces which they cannot access or alter. Single celled organisms have much more direct control over their interfaces with their environment.

Also that algae evolved to live in a dense soup of not only bacteria but unicellular predators all day every day, and have lots of very interesting responses on a cellular level to respond to attempts to eat them.






But it's actually very cool, turns out that far from being suffocated when they encounter an overgrowth of nitrifying bacteria, unicellular algae can bind up with them, create hybrid surface films and go to town:

The algal-bacterial symbiotic process uses the interaction between bacteria and algae to purify water.. Algae provide oxygen through photosynthesis, facilitating the oxidation of ammonia by nitrifying bacteria, while denitrifying bacteria utilize available carbon sources in the water, whether original or additional, to achieve nitrogen removal. Compared to conventional processes, the algal-bacterial symbiotic system can save approximately 46 % of oxygen required for nitrification

If you can't beat 'em join' em algae can team up with excess bacteria to adapt in real time.. leaving us and our plants in the dust lol.