Basic Understanding of Dissolved Oxygen in Freshwater

[Bradders]

Again, this is a first pass and a rudimentary level of testing.

Yesterday, the dissolved oxygen went to 7.25mg/l just before the photoperiod ended at 20:00. (NOTE: photoperiod is still only 6 hours at 25% lighting). This morning at 08:00, the DO remained at 6.10mg/l - which is safe and healthy level for fish.

I know this is obvious to the UKAPS experts (I am still learning by doing!), but it shows:

• How vital the plants are in building up 'oxygen reserves' during the day to carry through the non-photoperiod times.
• How highly planted aquariums with limited observable surface agitation or airstones can support a moderate amount of fish.
• How aquarists can run into (almost silent) trouble when plant mass is low, plants are dying/large algae outbreaks, and you don't have a lot of artificial aeration to compensate.

It might sound sad, but its all very interesting!

 

[dw1305]

Hi all,

Yesterday, the dissolved oxygen went to 7.25mg/l just before the photoperiod ended at 20:00. (NOTE: photoperiod is still only 6 hours at 25% lighting). This morning at 08:00, the DO remained at 6.10mg/l - which is safe and healthy level for fish.

Try it again at 40% intensity?

How vital the plants are in building up 'oxygen reserves' during the day to carry through the non-photoperiod times.

That is the point I've always tried to put over to the unconvinced, that plants are <"massively net oxygen producers">.

When I started on this journey I had expected that this would be the <"consensus amongst serious aquarists">, but I met a lot of (often really good) fish-keepers who have regarded plants as unimportant for either,

• oxygenation (so basically "at best, that they consume as much oxygen as they produce") or
• fixed nitrogen uptake (again "that they are, at best, very minor players when compared to the ammonia and nitrite oxidising filter bacteria, and that you need a huge amount of plants to process the waste from even one small fish and plants only uptake nitrate").

I knew this wasn't true, but I hadn't realised how ingrained those views were amongst <"ordinary"> (although some of them were extraordinary) fish keepers. I think this partially because of the narrative from the vendors of <"denitrifying filter media"> etc.

The <"internal storage of gases in plants">, CO2 from respiration during the dark period and oxygen from photosynthesis, is also an area that has been under-appreciated, and <"a rooted emergent plant"> would have been optimal for the <"Duckweed Index">.

cheers Darrel

 

[Bradders]

Try it again at 40% intensity?

It's only been running for 8 days, so slightly nervous about pushing it up at the moment! Thoughts?

That is the point I've always tried to put over to the unconvinced, that plants are <"massively net oxygen producers">.

Indeed Darrel. And this (very) rudimentary testing shows that for real. I was amazed at such healthy fish in a planted aquarium with almost a flat surface. I was also amazed at healthy fish (like common cichlid tanks) with a no/low plant mass and 'artificial' aeration techniques. But what really caught my eye was how the 'mechanics' worked when you have beautiful and heavily planted aquariums with fish AND absolutely minimal artificial aeration - sometimes the surface looked like it was hardly moving at all. The net oxygen production is that answer.

 

[dw1305]

Hi all,

It's only been running for 8 days, so slightly nervous about pushing it up at the moment! Thoughts?

Try it for a day and see what happens to the oxygen level. If you think the light is too bright (too much PAR) you can always go back to 25% or try a longer photoperiod at 25% etc.

I was also amazed at healthy fish (like common cichlid tanks) with a no/low plant mass and 'artificial' aeration techniques.

You can definitely have efficient oxygenation without plants. I've corresponded a bit with American "Monster fishkeeper"<" Viktor Jarikov"> and he has now, largely, solved his oxygen problems without using plants.

You don't really have a lot of choice if you tank is very lightly planted and heavily stocked. The more ammonia you have (it doesn't matter where it came from) the more oxygen you need to process it. If you haven't seen them you might be interested in the videos of Viktor Jarikov on PlanetCatfish, he is a Monster Fish keeper.

Plants as oxygen producers
I should have put in some links <"for this"> post, some of them are quite surprising:

oxygenation (so basically "at best, that they consume as much oxygen as they produce") or

Oxygenation from: <"Do Aquatic Plants Produce Oxygen For Fish In Aquariums? | Atlas Scientific">

....... During the day, aquatic plants absorb CO2, producing O2, and during the night they absorb O2 and produce CO2. However, you will rarely experience oxygen shortages during the night or dark phase of your lighting cycle, unless you are solely relying on aquatic plants as an oxygen source for your fish.

Most of the O2 in your aquarium will come from dissolved oxygen at the water surface, where the surrounding air can be diffused/dissolved into the water. Therefore, the amount of oxygen your aquatic plants produce is minute compared to the amount of O2 dissolved from the outside environment........

And they claim to be <"experts">. I'll try posting on their blog and see what happens*.

...... If you have any questions regarding dissolved oxygen, or you would like to learn more about other water quality measurements, characteristics, or applications for DO, please do not hesitate to contact our world-class team at Atlas Scientific....

I should say that I also like <"some water movement">. I see a <"large gas exchange surface to volume ratio"> as an unalloyed god thing.

*This, open access, paper would refute that view straight away <"Effect and Mechanism of Applying Myriophyllum Verticillatum for Reclaimed Water Purification in Urban Rivers"> Zhu, L.; Lin, X.; Li, S.; Sun, D.; Fang, H.; Xu, J.; Huang, J.; Li, G. Effect and Mechanism of Applying Myriophyllum Verticillatum for Reclaimed Water Purification in Urban Rivers. Water 2023, 15, 2331.

..... According to the results of the pre-experiment, the plant densities of the treatment groups were set to 1.5 g L−1, 2.0 g L−1, 2.5 g L−1, 3.0 g L−1, and 3.5 g L−1 by wet weight (WW), and no submerged plants were placed in the control groups.

Nitrogen

...... fixed nitrogen uptake (again "that they are, at best, very minor players when compared to the ammonia and nitrite oxidising filter bacteria, and that you need a huge amount of plants to process the waste from even one small fish and plants only uptake nitrate").

Their section on nitrogen uptake is better, and generally the fact that plants take up all forms of fixed nitrogen has definitely been more widely disseminated in recent years: <"Is Nitrate Good or Bad for Your Aquarium? | Water Chemistry 101">.

..... The biggest benefit to adding aquatic plants to your aquarium is that they produce oxygen and remove carbon dioxide (CO2) and ammonia (NH3) produced by your fish. .......

cheers Darrel

 

[Bradders]

This has been a fascinating journey @dw1305. I like it when things click into place between experience, research and some good old fashion testing!

 

[Bradders]

Oxygenation from: <"Do Aquatic Plants Produce Oxygen For Fish In Aquariums? | Atlas Scientific">

And they claim to be <"experts">. I'll try posting on their blog and see what happens*.

*This, open access, paper would refute that view straight away <"Effect and Mechanism of Applying Myriophyllum Verticillatum for Reclaimed Water Purification in Urban Rivers"> Zhu, L.; Lin, X.; Li, S.; Sun, D.; Fang, H.; Xu, J.; Huang, J.; Li, G. Effect and Mechanism of Applying Myriophyllum Verticillatum for Reclaimed Water Purification in Urban Rivers. Water 2023, 15, 2331.

This is very interesting, and I think you should post on the blog to see what thier response is.

My experiment does seem to heavily suggest that a good build-up of DO during the photoperiod provides good levels for non-photoperiod. That being said, I dont have any fish in that aquarium whatsoever at the moment. So not quite sure how that will change if I insert five 4-inch angel fish into the mix. Do we have any idea of the oxygen calculation for varying fish over a time period?

 

[Bradders]

I did find loads of research on fish DO consumption, all as confusing as one another due to variables. One piece said the below, but I cannot determine how they got to the 0.84mg/l per 24-hour mark!

Average oxygen consumption rates for adult fish usually are between 200 and 500 mg oxygen/kg fish/hour. Assuming an oxygen consumption rate of 350 mg/kg/hour in a 1-ha pond with 1 m average depth, each ton of fish would use an amount of dissolved oxygen equal to 0.84 mg/L in 24 hours.

 

[dw1305]

Hi all,

I like it when things click into place between experience, research and some good old fashion testing!

That is it really. @Happi posted how they got around the issue of replication in <"My thoughts so far">.

I know that we all tend to have <"faith positions">, usually based on our experiences, but they aren't always optimal, or we might not understand why something <"new to us"> works.

If we can use experimentation and research to increase the <"sum of human knowledge">, or just make aquarium keeping easier and more pleasurable, we've done a good job.

cheers Darrel

 

[dw1305]

Hi all,

Average oxygen consumption rates for adult fish usually are between 200 and 500 mg oxygen/kg fish/hour.

It is going to vary between fish, I'm going to assume the top figure is an active carnivorous fish, something like Atlantic Salmon (Salmo salar), and the lower figure a more sedentary fish, something like Channel Catfish (Ictalurus punctatus), Tilapia (Oreochromis niloticus) or Common Carp (Cyprinus carpio) etc.

I did find loads of research on fish DO consumption, all as confusing as one another due to variables. One piece said the below, but I cannot determine how they got to the 0.84mg/l per 24-hour mark! "Assuming an oxygen consumption rate of 350 mg/kg/hour in a 1-ha pond with 1 m average depth, each ton of fish would use an amount of dissolved oxygen equal to 0.84 mg/L in 24 hours".

The 350 mg oxygen/kg fish/hour is just a middle value from the range above, and the dimensions of the pond are just to make the arithmetic easier.

One hectare is 10000 m^2 , the depth of the pond is 1 m so we can ignore that (10000 m area x 1 m depth), the density of water is 1 (kg / L) and a m^2 of water is 1000 kg (and 1000 litre), so you have 10,000 x 1000 = 10,000,000 litres (kg) of water in the pond.

Then we have 350 mg O2 used per kg and a 1000 kg in a tonne, so the tonne of fish use 350 * 10000 mg O2 per hour = 350,000 mg O2 per hour.

Then you just need to sort of the "figures of ten" (350,000 / 10,000,000) to 0.035 and 0.035 * 24 (hours) = 0.84 mg / L.

cheers Darrel

 

[sparkyweasel]

"And they claim to be <"experts">." in what? 🙂

 

[dw1305]

Hi all,

"And they claim to be <"experts">." in what? 🙂

Electronics and aquaculture apparently.

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Cheers Darrel