Help me understand DOC - Dissolved Organic Compounds - The Planted Tank Forum
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post #1 of 5 (permalink) Old 02-23-2019, 05:59 PM Thread Starter
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Help me understand DOC - Dissolved Organic Compounds

There's something I just don't understand about DOC. Waste, fish poop or dead plant matter or whatever, is acknowledged as an eventual source of plant food. It needs to be broken down by various bacteria and other organisims first, yes, but eventually poop in - NPK out. Maybe it happens on the ground in tank, or maybe it happens in your filter, but it happens. Everywhere right?

Is not DOC just very small free floating waste? Is it not, just like any other waste, on it's way to becoming a plant nutrient? How is there such a thing as excess DOC that would need to be removed by water changes? Is not DOC just a tiny bit of waste that came off of a larger bit of waste in the filter or in tank? If so, one assumes it's now super bioavailable to the bacteria/critters turning it into plant food. The smaller it is, the more surface area for benificial bacteria to consume.

But for some reason it's bad. Contributes to algae. Don't algae consume the same things as plants? If they can start consuming waste BEFORE it's finally decomposed enough to be turned into plant food, then i see a problem. But then, I see a problem for waste being considered a source of plant food and a need to totally eliminate waste from the water.

I'm sure someone will try to explain this with the concept of EXCESS. You don't want excess waste. But I've considered that and it doesn't make sense either. All waste needs to go through a process to become plant food. So first you need excess waste. In a mythical ideal tank that produces the exact amount that's consumed you first need a pile of waste to begin producing for the consumers. In order for there to be a large amount of consumers, there also needs to be a large amount of producers, there needs to be A LOT of waste, and as follows, a large amount of DOC, on it's way to becoming food for the consumers. Certainly, waste is a problem in tanks with insufficient consumers, but why is it a problem in planted tanks with an overabundance of consumers, so many that supplemental fertilization is necessary?

A long way of asking why is DOC bad? I've actually read a fair bit on the subject, but still seem to be missing something.

Bump: I've found lots of things on this subject, things like this: http://www.aquascapingworld.com/magazine/May08/Dissolved-Organic-Compounds-Explained.html

And forum posts around the internet similar. But nothing that explains WHY bad. At least to my satisfaction.

Bump: I've found lots of things on this subject, things like this: http://www.aquascapingworld.com/magazine/May08/Dissolved-Organic-Compounds-Explained.html

And forum posts around the internet similar. But nothing that explains WHY bad. At least to my satisfaction.
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post #2 of 5 (permalink) Old 02-23-2019, 06:28 PM
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As I understand it, organic compounds are considered bad because they still have significant food energy in a form accessible to a wide spectrum of microorganisms. The microorganisms feast on the stuff, and, in the process, they consume oxygen and convert carbon in the waste to carbon dioxide, nitrogen to ammonia, and phosphorus to phosphates. These are, of course, all plant foods, as you have realized. It's the consumption of oxygen and proliferation of the wrong kind of microorganisms that can potentially cause problems. These may include algae that are able to use the carbon in the DOC directly.
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post #3 of 5 (permalink) Old 02-23-2019, 06:57 PM
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I get asking the question, but to me the "Why" doesn't matter that much.

The important thing is to me is that I see a clear relationship between algae and tank cleanliness. Algae seems to love a tank with too many dissolved organics, and it seems to hate uber clean conditions.

We talk a lot about balancing light/CO2/Ferts, but maintenance and clean conditions are equally important.


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post #4 of 5 (permalink) Old 02-23-2019, 10:10 PM
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Iíve always found this an interesting topic. Before we go on, be sure you understand that the organic stream from fish food and plants does not contain potassium. You mentioned NPK as the end-result of organic conversion, but only N and P are involved, not K.

I donít view DOCís as bad. They are broken down into the inorganic NH3/NH4 and orthophosphates that plants (and algae) can use. In fact, I have no bio-media in my filter so that my plants donít have to compete with the BB for the NH4/NH3. The BB in the sediment and coating surfaces do not have the high access to the NH4/NH3 that the BB in a filter do, simply because the filter is constantly providing nearly immediate access to the NH4/NH3. So, my plants have better exposure and, therefore, use more of this stream when there is no bio-media in the filter ÖI assume. If I could, Iíd completely eliminate all DOCís instantly and just add NO3 and PO4, which would probably eliminate algae, but you canít, so you have to remove as much as possble either by cleaning or plant consumption.

However, depending upon the bio-load, feeding, rotting/unhealthy plants, and even tap water, there can be more of these DOC-created inorganics than plants and BB can consume. So, as they break down into inorganics, they feed algae. Algae feed mainly on ammonia and orthophosphate. Tests have shown that NO3 does not induce algae. The fact is that you cannot get DOCís out fast enough to eliminate the tiny amount that algae can use. However, if itís more than plentiful, algae can explode. Keep the tank clean and you will generate fewer DOCís. Change the water and you can help further, but they will build-up between w/cís.

You might find these two articles interesting:

https://www.advancedaquarist.com/2002/9/chemistry

https://barrreport.com/articles/nitr...-june-2005.20/
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post #5 of 5 (permalink) Old 02-24-2019, 12:37 AM
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Yes organics can provide nutrients N and P as well as Ca, Mg, Fe, Mn, Zn, Cu, Mo, Ni as well as other elements. However some nutrients may be lacking in fish waist. K, Cl, B, S can easily convert into a gas or are very water soluble compounds that are easily removed in a water change. So fish waist may not be a balanced source of nutrients.

Additionally 90% of fish waist is carbon, oxygen and hydrogen. When bacteria consume these oxygen can be depleted in the water. Fish and plants don't do well in oxygen depleted water. Algae does very well under those conditions. Additionally some organic are acidic and will affect PH and KH which may be good and bad, However once the organics have been completely consumed all that is left is CO2 which plants as well as Algae like.

Overall organics can make the tank chemistry unstable. And stable chemistry is needed to maximize plant growth while minimizing algae growth. I have seen examples of tanks that do well with good plant growth and minimal algae with considerable waist. If you can maintain that condition it's good. But often that can't be done. So frequently waist needs to be removed.
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