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High DOC, similar to high nitrate, is an indicator of general pollution, as some compounds are toxic to fish at high concentration. I also read that high DOC can trigger algae, so heavily stocked tanks can become an algae farm unless DOC are removed by Purigen as recommended. Barr has done an experiment to add more and more minors to a planted tank and eventually algae popped up.
I've found an interesting reading that shows how essential DOC can be for fish survival under specific conditions.
For example, Amazonian Rio ***** is a very acidic environment, and blackwater DOC protects fish from going belly up in this harsh environment.
https://www.nature.com/articles/srep20377
This source also points out that DOCs are not created equal as you mentioned. There are also lots of papers that discuss how DOC reduces toxicity of Copper, Aluminum, Zinc etc for fish (and shrimps)...

I can also see that DOC can be toxic. Say if I dumped a bottle of vinegar into the tank, I am sure fish would not be that happy.
 

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I've found an interesting reading that shows how essential DOC can be for fish survival under specific conditions.
For example, Amazonian Rio ***** is a very acidic environment, and blackwater DOC protects fish from going belly up in this harsh environment.
https://www.nature.com/articles/srep20377
This source also points out that DOCs are not created equal as you mentioned. There are also lots of papers that discuss how DOC reduces toxicity of Copper, Aluminum, Zinc etc for fish (and shrimps)...
Wonderful paper! Thank you for bringing it to our attention. One more reason to love DOC. :)

Natural DOC compounds are produced by the decomposition of plant matter. DOC is not an indicator of pollution. The only somewhat harmful DOC would be organic acids (acetic acid) released via fermentation in severely anaerobic substrates. They will inhibit plant roots.

DOC is a vital source of CO2 for NPTs. When you remove DOC, you remove a natural source of CO2. What's nice is that unlike artificial CO2 injection, the decomposition of DOC occurs at a slow and gentle rate. Won't kill the fish.

Hobbyists with high tech tanks remove all the plant nutrients with water changes, Purigen, excessive filtration/aeration. Then, they put them all back--at great cost--with fertilizers and CO2 injection.
 

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DOC can mean different thing in different setting. DOC in natural tannin tainted water is not harmful but beneficial to local fauna as it is a product of decaying vegetative matter. Blackwater fish keepers often use peat filtration to simulate tannin water to promote healthy growth and breeding. Amazon blackwater is actually low in DOC except that it is dominated by tannin. In urban or agricultural lands, high DOC typically means human pollution from runoff that can lead to eutrophication or oxygen depletion threat in low flow conditions as high DOC equals high BOD. In a NPT, some DOC is good as a source of carbon and nutrients for plants, but too much DOC often means over feeding, over stocking or lack of WC that is bad for plants and fish. A balanced NPT has light stocking but many hobbyists, like me, overstock.
 
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