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COD

I'd like to add some things about COD.

First point is a question of definition
COD is the quantity of oxygen which will be add from a very strong oxydising agent which is K2Cr2O7 to a sample of water in order to oxyde all that can be oxydised bi this agent. It include most organic material but also mineral reduced material as Fe²+, sulfur, etc, ...
But COD is a measure with a very powerfull oxydising agent. Only a part of the material which create COD can be biologicaly attacked in a time of a few days. It include most of organic material but not all. For example some humic substances won't be naturally oxydised before years whereas they are count in COD. So those substances represents no danger for the aquarium regarding oxygen concentrations.

It's a big mistake to say "my aquarium as got 8 mg/L O2; my COD is 10 mg/L so my fish will die".
O2 in aquarium is ad constantly owing to photosynthetis but also owing to water movement (air as much more oxygen than water can contain so if water is not saturated O2 will tends to go into the water).
We saw above that not all COD will consume oxygen in a tank but also we can say that it will take days and days to consume most of COD into the tank. So O2 will be replaced in the tank really faster than it will be consume to decrease COD.

about accuracy :
The COD was introduced for waste water treatment. So it is accurate for quite high levels of COD (approximatly more than 20 mg O2/L) but official mesure method won't work well below. By the way it is also used at low values but I think it can't be considered as very accurate (this mean that when you read a COD of 4 mg/L it could also be 3 or 5 mg/L). And more in "accurate" range (more than 20 mg/L) the measure precision is maybe around 5%. For example when you test the same sample of waste water for COD you can obtain 300 mg/L for one measure and 310 for the second one.

COD will only give indication about water quality but you must be very carefull about the signification to give to the values.

At the present time there is a project in European Union to evaluate rivers and lake quality. The classification is based on many parameters including COD.

For COD water are classed as :
very good : COD below 20 mg O2/L
good : COD between 21 and 30 mg/L
average : COD between 31 and 40 mg/L
bad : COD between 41 and 80 mg/L
very bad : COD more than 81 mg/L

I think that water in aquarium should always be "very good".

I hope it is clear enough. it's very hard for me to write such things in english.

EDIT :
TH mesure water total hardness (it is french notation, same as german GH).
1°GH = 1,78°TH

E. Coli is search as an indicator of fecal contamination for statistical reason : it is the most often pathogenic organism seen
 

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Discussion Starter · #22 ·
Anthon,

That is a very informative post on COD. Thanks for taking the time to add it. Being young in aquahobby experience years (and old in real years), the additional explanation helps a lot.

My simplistic understanding of the Amano-type tank that inspired this discussion is that there is a focus on water quality maintenance - as well as plant growth sustenance. Being new to the hobby, I spend much of my time worrying whether the CO2 level is too high, too low, or is not being consistent enough from day-to-day. I have not thought of the very important aspect of oxygenation - apart from fish respiration requirements. But I am coming to think that is at least as important as CO2 fertilization in an aquatic garden.

Last night I was reading the Kesselman book, Aquarium Plants, the introductory materials. Kasselman does put some emphasis on oxygen needs of the whole aquarium - water, plants, bacteria, substrate etc. That book is expensive - but worth a read.

I suspect that the Amano tanks put some focus on that as well. The water returning to the tank overnight is aerated. Glancing through the Net articles and groups etc, one can conclude that it is CO2 that we put into return filter water more often than not - but never aeration. Amano uses simple (if elegant and expensive glass) diffusers for CO2 addition - not in-line reactors on the return filter tube. From this alone, I think that their philosophy places almost as much attention on O2 and oxidization potential for cleaning water as it does on CO2 addition for plant growth in the day time.

Just as a beginner, I have adjusted my thinking away from a focus on CO2 addition to a general focus on water quality, and that would include reasonable aeration if that can aid water quality improvement or maintenance. The water is the medium....

In the few aquariums we run, adding aeration at night has helped a great deal - both in terms of bettering water quality and even perhaps in plant growth. Oxygen meters are way to expensive to buy so I am working only from simplistic observations. Maybe I am speaking baloney - apologies to the skilled among our brethren if so.

Andrew Cribb
 

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pineapple said:
Just as a beginner, I have adjusted my thinking away from a focus on CO2 addition to a general focus on water quality, and that would include reasonable aeration if that can aid water quality improvement or maintenance. The water is the medium....
Andrew, emphasis on water quality is generally a good thing but don't let that emphasis lead you to make conditions in your tanks worse. The most direct route to higher water quality in an aquairum is to reduce fish loads and fish feeding, remove decaying material and maintain large regular water changes.

The oxygen level in a healthy planted tank is generally higher than the 8 ppm or so the water would get from aeration. This is true in tanks with low or moderate fish loads even if you don't add CO2 or have very bright light. If you aerate the tank during the day then you will only decrease that oxygen level. If you aerate it at night you will cause the oxygen level to drop faster than it would without aeration. Aeration at night might maintain oxygen levels at significantly higher values then you would get without aeration, but that will depend on a number of other factors.

When I had an oxygen test kit (years ago) I found that my moderately-populated and dimly (by current standards) lit 55 gallon tank would peg the scale of the test kit at over 10 ppm during the day. That was without whatever added boost might come from CO2. When the lights went out the O2 levels would drop back to 8 or so by morning. That tank had good surface agitation and that prevented the oxygen levels from dropping lower. Additional aeration would not have improved conditions.

I have to wonder how (and why) one would aerate through a lillie pipe. Could Amano have meant that the water was circulated at night and aerated by the circulation?

Roger Miller
 

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Re: COD

Anthon, thanks for your post. It is very interesting.
Anthon said:
It's a big mistake to say "my aquarium as got 8 mg/L O2; my COD is 10 mg/L so my fish will die"
. I agree that statement would probably not be right but I prefer to think of it as safely conservative rather than as a big mistake :)

Roger Miller
 

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Discussion Starter · #25 ·
Roger,

emphasis on water quality is generally a good thing but don't let that emphasis lead you to make conditions in your tanks worse.
That is a good point. I imagine vacuuming my modest tanks would be somewhat easier than doing the same to the Amano exhibition tanks. I do aim for a modest fish load, being more of a plant-focused person, and a modestly fed population too.

There are many enjoyable details to observe in an aquarium and each can distract one from something, perhaps, important: algae growth or decline; plant growth and colouration; smell of water, surface film, clarity; and more. All these things occupy attention. I am finding it a little more productive to focus that attention on a general philosophy of water quality. But as you say...

If you aerate it at night you will cause the oxygen level to drop faster than it would without aeration. Aeration at night might maintain oxygen levels at significantly higher values then you would get without aeration, but that will depend on a number of other factors.
True enough. I am wondering if 'aeration' and 'oxygenation' are terms that should be used more cautiously. Possibly aeration is, as Tom said, more related to water movement. Air is not only O2 either.

Additional aeration would not have improved conditions.
Possibly in an early stage of plant growth in an aquarium it would have been more helpful.

I have to wonder how (and why) one would aerate through a lillie pipe. Could Amano have meant that the water was circulated at night and aerated by the circulation?
Much of my business is with China and Taiwan. My colleagues and I (both Chinese and non-Chinese) laughingly talk about 'Chinglish' - Chinese-English. I suspect there is a Japanese equivalent too and that one might have to be careful in interpreting the English. The reason Amano tanks use a Lily Pipe is that it makes for a gentle strong flow which seems not to cause very much surface disturbance. Perhaps - if they aerate return water - that is why they aerate it...

I think we will have to have an APC outing to Nigata and investigate ourselves.

Andrew Cribb :wink:
 

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Re: COD

Roger Miller said:
Anthon, thanks for your post. It is very interesting.
Anthon said:
It's a big mistake to say "my aquarium as got 8 mg/L O2; my COD is 10 mg/L so my fish will die"
. I agree that statement would probably not be right but I prefer to think of it as safely conservative rather than as a big mistake :)

Roger Miller
You know many rivers have more than 20 mg/L COD and 20 mg/L is much more than O2 concentration as if water is saturated in O2. Hopefully fishes don't die for that. But it is true that a low COD is much more preferable for aquatic life.
 

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I don't know that redox plays in here at all. The redox potential depends on the existence of oxidized and reduced couples that are reactive enough to register on the electrode. And it depends on having a useful electrode. I've dealt with them professionally and found redox results to be only roughly related to chemical conditions.


Roger Miller
 
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