I want to add the question, how it works wih semi-emersed plants. When they access co2 above the surface, do they add o2 to the water?

 

Welcome to APC!

I don't know of any measurements of dissolved oxygen in tanks with floater compared to tanks without floaters. In my own tanks I try to keep enough floaters to encourage normal fish behavior and control light intensity, but no more than that. If I am neglectful and let the floaters grow to a thick cover over the whole tank, fish sometimes show distress. Removing at least half the floaters and making sure there is enough water circulation solves the problem.

 

I think you're asking several different, but related questions. One is whether floaters themselves contribute O2 to the water column and I think the answer is perhaps a bit via their roots, but not nearly enough to be considered major contributors of O2 to a tank of water. Submersed plants do most of the heavy lifting and it sounds like you have an ample amount. The other question is whether too many floaters will interfere with the surface exchange of gases between the water and air, and that's a genuine concern, IMO. Letting floaters completely cover the top of your tank is probably not a good idea.

 

Well said!

I would add that the ratio of surface area to water volume is also important. I don't run aeration in any of my tanks, but they are shallow (12" or 30 cm) and well-established (soil oxygen consumption has declined to a low level). The tanks also have a lots of ground cover (Subwasstertang, thread algae, and ferns that generate oxygen. The tanks all contain floaters, but I keep the cover down to one half to two-thirds.

BTW, I found that a stiff bristle brush is great for removing small duckweed from the surface. Some tanks I have to clear out the duckweed every few days!

 

My recomendation is that you should not really only on the plants producing enough oxygen for your fish.

During water changes pouring water into the tank has occasionally pushed floating plants under the surface and occasionally they have gotten entangled another plants and trapped below the surface. when the light came on oxygen produced from the plant collected on the top side of the leaves of the floaters. Most submerged plants rooted in the substrate have oxygen bubbles form under the leaves.

This indicates to me that the stomata (the holes that plants use to get air into and out of the leaves) are all on the top side of leaves of floating plants. So most of the oxygen they produce will go direct into the air not into the water.

Also biological activity in the substrate and filter may consume considerable amounts of oxygen. It that is happening in your tank when the lights are off oxygen levels could drop critically low levels that may impact the health of your fish at risk at night.

 

Also biological activity in the substrate and filter may consume considerable amounts of oxygen. It that is happening in your tank when the lights are off oxygen levels could drop critically low levels that may impact the health of your fish at risk at night.

If bacteria were a significant factor in O2 consumption, that would be one more argument against keeping a biological filter attached to one's tank. As it is, I would be extremely surprised if the biggest consumers of O2 in a NPT were not the fish themselves.

The important thing to remember is that a heavily planted tank is capable of outcompeting bacteria for important nutrients and to not let decaying leaves and debris accumulate at the bottom of the tank where bacteria will use up 02 in breaking down the cellulose (snails and shrimp are a big help in keeping food and fuel away from bacteria, too.)

 

Thanks to everyone that replied. I think I was actually asking several different questions...or to put it another way, trying to get at "How can I establish and plant my tank in such a way that adequate oxygen is created?"

I think what I heard in response is 1) plant as fully as possible and 2) stop worrying so much about what floaters do and do not do (assuming you have #1 covered), but don't too crazy with them.

I'm a good ways down the road on #1 - I think my plants will be fully established with an advantage over algae in a couple weeks. ...and #2 is good and easy advice to follow.

Thanks all!

 

Well, the original question was whether floating plants contributed O2 to the aquarium and the consensus seems to be, "not really." Someone else suggested that bacteria consume O2 and therefore compete with plants and animals for dissolved O2 in the water column. I don't think that’s true. My understanding, based on EPA pp. 57-58, is that bacteria get their O2 from organic and inorganic compounds, not from the water column. The confusion, I think, lies in the fact that decomposition of organic matter results in CO2 which can displace O2 in the water column, if produced in sufficient quantities.

But this is way beyond the scope of the discussion starter's original question.

 

Well, the original question was whether floating plants contributed O2 to the aquarium and the consensus seems to be, "not really." Someone else suggested that bacteria consume O2 and therefore compete with plants and animals for dissolved O2 in the water column. I don't think that’s true. My understanding, based on EPA pp. 57-58, is that bacteria get their O2 from organic and inorganic compounds, not from the water column. The confusion, I think, lies in the fact that decomposition of organic matter results in CO2 which can displace O2 in the water column, if produced in sufficient quantities.

Floating plants, like ALL plants, release some photosynthetic oxygen from their roots. However, a thick floating plant cover will block oxygen diffusion from air into the water, plus a thick mass of floating plants will often contain some decomposing leaves/roots that would will consume O2. So the O2 contribution by floating plants column is not a given and is less than submerged plants.

Most bacteria get their O2 from the water, which they use--just like us humans-- to metabolize/process organic compounds (i.e., food). Nitrifying bacteria use oxygen in a different way, but they still use lots of it as they oxidize ammonia to nitrites and nitrites to nitrates. All these bacteria compete with fish for oxygen in the water column.

And, please, CO2 does not displace O2 in the water. These two gases run on separate tracks. (They act independently of each other.)

Glad people are reading my book, but it seems sometimes I need to clarify certain points.

 

Don’t forget plants consume O2 at night.

 

Yes and if I'd read a little further, on page 59, I would have discovered that you'd said as much:
"Aerobic decomposition, which requires oxygen, is much faster than anaerobic decomposition. Thus, air/water mixing and plant photosynthesis stimulate decomposition by adding oxygen to the water." (p. 59, EPA)
Thank you for the clarification!

Is it fair to say that anytime an organism produces CO2 there's a separate and equal consumption of O2?

 

Is it fair to say that anytime an organism produces CO2 there's a separate and equal consumption of O2?

Yes, indeed! (The same oxygen atoms in O2 found in water column go into CO2 during decomposition (i.e., the oxidation of organic matter). The same oxygen atoms in O2 in water column go into nitrate during nitrification (the oxidation of ammonia.)
Chemistry should make sense, and it does!

 

Just wanted to say thanks to everyone that responded. This is helpful.