O2 Saturation in Planted Tanks

[Jchillin]

This subject has come up a few times. I figured there's a chance that a full understanding is neccessary in order to help our fellow aquarists on this subject.

This is the present theory: In a planted tank, surface agitation depletes the water of CO2, which the plants require. Your filter (whether HOB or canister) should be set up with little or no agitation. Plants give off O2 as a by-product of photosythesis, so aerating the tank isn't really necessary.

One of the issues: If you are "lightly planted", and utilize the above method of filtration, is your tank getting enough O2? Does this take into account whether you are lightly or heavily stocked with fish?

The other issue: If you are using a canister and the outtake is submerged, this results in a semi "closed" loop (water taken into canister via the hoses and being returned with little or no aeration). Is the amount of aeration enough? Does it matter with the size of the surface area and/or stocking levels?

I know this has been discussed, probably in great detail. Just some thoughts and nothing more.

Thanks.

 

[greenmaji]

Just a point to ponder.
As far as surface agitation being bad I was under the impression that surface agitation was only bad in CO2 injected tank..
I thought that if your not injecting CO2 then surface agitation was greatly encouraged (or at least it is implied that it would be better to have more agitation to get more CO2 into the water from the air in the room)

and as for injected tanks..
plants consume O2 at night and I have read about bad die-offs from fish suffocating at night when people are running there CO2 without a controller at night.. So I do wonder whats up with this myself..

 

[malkore]

Here's a few facts to start this off:

O2 saturation in tropical temperature water: ~4ppm
CO2 saturation in tropical water: well over 100ppm

Surface agitation doesn't necessarily deplete CO2...it simply removes the excess, taking you down to 'normal' levels for water, which is about 3-4ppm. It also helps maintain the ~4ppm of O2.

I have zero surface agitation in most of my planted tanks, particularly my 75gallon which uses a canister filter. However its a high light tank so it doesn't make a good example. My 5gallon tank is low light, with an internal duetto filter for zero surface agitation...however I have a betta in there, so O2 levels in the water are fairly unimportant to him.

Here's my take: You would need a very heavily stocked/overstocked tank, low light, with zero surface agitation, and next to no plants, in order to create an O2 deprived environment. Even at low light, if you had tons of low light plants, they'd create enough O2 for most situations. Just because you cannot see pearling doesnt' mean the plants aren't producing oxygen...it just means your water hasn't hit O2 saturation. O2 saturation won't allow the respired O2 from the plants to dissolve into the water, so instead it forms into 'pearls' of pure oxygen that float to the surface.

My 75 is well stocked, so I don't see heavy pearling anymore, and some days, zero pearling.

 

[greenmaji]

After reading the article by Takashi Amano in the July issue of Tropical Fish Hobbiest I noticed in his equipment list for the tank featured in the photo's that it seems he is injecting air just before the lights are turned back on.. this seems to be counter to the no agitation argument..

 

[Jchillin]

Taking all of this into account, would it be prudent to test O2 and CO2 levels in either one (low-light...lightly planted, heavily stocked/low-light...heavy planted...light-low stocked)?

You would need a very heavily stocked/overstocked tank, low light, with zero surface agitation, and next to no plants, in order to create an O2 deprived environment.

Indeed. This is very important. Probably the best description of why this issue comes up.

 

[malkore]

Well it's always good to know your CO2 levels if you are injecting it. If it's a low light tank, there's no reason to bother with it, because you don't need extra, and you won't have extra.

As for O2 levels, I'd say if you've got that low light, heavy stocking situation (regardless of number of plants), that would be the time you might wanna invest in a simple O2 test kit. However I would probably only do this if I had more than 1 unexplained fish death, if the fish seemed lethargic, or if they appeared to be gasping for air and hovering over at the top of the tank, especially if they are mid or bottom dwellers.

One thing I did forget to mention, was that CO2 does not push oxygen out of the water. its totally possible to maintain O2 saturation even with CO2 levels at 100ppm. A lot of people thing the two are balanced somehow. I'm sure if you actually hit CO2 saturation, you might not be able to have as much O2, but you'd kill all your fish before hitting CO2 saturation, so its a moot point really.

Also I dug around online to find the saturation level of CO2 in water, but since pressure and temperature have such a huge effect on saturation levels, I didn't find anything concrete. I did, however, learn that atmospheric levels of CO2 is ~350 ppm! You would need a dKh of 5 (ballpark average) and enough CO2 to drop your pH to 5.7 in order to hit that level. Fish would start to die at a pH of about 6.4, which would be 60ppm of CO2. But they aren't dying from CO2 poisoning, they're dying from the acidic conditions of the water, caused by the CO2 levels. I doubt they'd live to pH 6.0

Since my tank is 10 dKh, my fish are fine at 85ppm of CO2, because that's only a pH of 6.6

 

[Jchillin]

Fantastic! Thanks Malkore...you've solved a riddle which has been wrapped in an enigma.

Kudos coming your way!

 

[Codefox]

So, in conclusion, is aggitation good or bad then?

 

[travis simonson]

After reading the article by Takashi Amano in the July issue of Tropical Fish Hobbiest I noticed in his equipment list for the tank featured in the photo's that it seems he is injecting air just before the lights are turned back on.. this seems to be counter to the no agitation argument..

I do the same thing on my 125G. I've got a six foot bubble wall along the back of my tank that goes on when my lights go off. I'm not sure if Mr. Amano and I do it for the same reasons but I've noticed that, without oxygenation during lights out, my O2 levels drop to near 2ppm over the course of the night and I find my fish gasping at the surface in the morning when the lights come on. This is partially due to my planting and fish stocking levels (both high), although I do run two Filstar XP3 canisters for filtration. I've also been supplementing with a photosynthesis precursor chemical that has effects that are similar to (but stronger in almost all aspects) Flourish Excel. This also has a detrimental effect on O2 levels but it does such wonderful things for the plants (and terrible things to algae ) that using nighttime oxygenation is not a problem if that's what it takes

 

[Jchillin]

I had added a bubble wall in my 75 gal for that very reason. I was under the same impression and it has worked very well. Though I did it for all the opposite reasons. I had felt that the canister output being submerged was depriving the tank of O2 at all times.

Since then, I keep it on mostly at night.

 

[Steve Hampton]

O2 saturation in Freshwater with zero salinity and zero chlorinity at 75F is 8.1 ppm.

O2 levels are largely influenced by the BOD (Biological Oxygen Demand) rate of your tank. Leaving uneaten food, ammonia/nitrite spikes, high bio-loads all require O2 to oxidize potentially harmful byproducts into less harmful or safe compounds. Certainly keeping water flowing and including surface agitation can insure that O2 levels remain high...levels below 6 ppm should be addressed. In my CO2 injected tanks the test kits will max out (10 ppm) when photosynthesis is at full blast.

As was mentioned, more often it is low O2 levels that cause grief rather than high CO2 levels. Gasping is a sign of O2 stress, not CO2. CO2 stress is evidenced by head standing and laying on the substrate with rapid gill movement. When CO2 stress actually occurs will depend on O2 levels and some fish are acutely more sensitive. Essentially when the CO2 levels rise in the tank above those in the fishes blood their gills are no longer able "pass" the CO2 into the tank water and acidosis sets in...it's frightening to watch. So it is really the differential between the CO2 inside the fish and the level in the tank that causes a problem. There are also some physiological issues related to lower O2 levels and high CO2 levels but thats another long post.

CO2 injected tanks should never have issues with CO2 stress, as long as you don't eliminate water circulation and surface turbulence. This is counterproductive on two issues. One is the possibility of lowering of O2 levels and two...it makes the Prandtl boundary more difficult for CO2 and nutrients to break.

IME one of the most important tasks for a DIY yeast CO2 tank is providing the right balance between trying to retain enough CO2 and not compromise O2 levels. It's my opinion that one should never sacrifice O2 for CO2...unless it's a plants only tank.

Just as a sidebar...Malkore mentioned atmospheric CO2 levels are approx 350 ppm, but that is outside. And it should be noted that outside (in air) it is measured by volume..it's a gas, and in water its concentration is measured by weight. If you compute this using the laws of partial pressure you'll arrive at about 0.5 ppm of CO2 in water at equilibrium at about 75F. However, it's common for levels to be near 1000-1100 ppm in a new well sealed home with the windows and doors shut...that takes us to just 1.0 to 2.0 ppm of CO2. The rate we often quote is 2-5 ppm for ambient CO2 levels...the rest comes from the error in the computing formula, biological processes, and test kit error. All this leads to why we quote such a wide range to target for CO2....more than you wanted to know, eh?

Another common question should be, "what are CO2 levels like in places where are plants are from"? One of the most insightful articles ever published in TAG listed the levels of nutrients from the Northern Pantanal...here's a partial list

NO3 - 4.6 ppm
PO4 - 0.19 ppm
K - 5.7 ppm
Fe - 0.06 ppm
SO4 - 0 ppm
O2 - 90%
CO2 - 11.4 ppm

All testing was done by Dr. G. Ritter, Tetra Werke Laboratory

 

[malkore]

Here comes Steve, always showing off

Looks like the source I found that said O2 saturation was 4ppm was a bit off.

 

[Jchillin]

more than you wanted to know, eh?

You aren't kidding However, knowledge is power and since we want to do things the right way...having this information is invaluable...so, kudos to you Steve.

Here comes Steve, always showing off

Isn't there always one in every crowd?

Once again...thanks so much and kudos to all of you.

 

[RoLeX]

greenmaji wrote:
After reading the article by Takashi Amano in the July issue of Tropical Fish Hobbiest I noticed in his equipment list for the tank featured in the photo's that it seems he is injecting air just before the lights are turned back on.. this seems to be counter to the no agitation argument..


I do the same thing on my 125G. I've got a six foot bubble wall along the back of my tank that goes on when my lights go off.

Wouldnt this cause large undesierable ph swings,

for eg, if u have co2 levels of 20ppm during the day and then u turn ur bubbler on at night, it would throw it back down to 4 ppm in a couple hours causing a ph swing of about 0.5 to 0.9. I havent had pressurized co2 myself yet, but all the thoery that ive read isnt this true?

Can someone give me their co2 reading at night and then in the morning after using an airator

Ta (as always correct me if im wrong, im here to learn)

Rolz.

 

[travis simonson]

Wouldnt this cause large undesierable ph swings,

for eg, if u have co2 levels of 20ppm during the day and then u turn ur bubbler on at night, it would throw it back down to 4 ppm in a couple hours causing a ph swing of about 0.5 to 0.9.

I use a pH controller which keeps the CO2 levels steady, although it likely requires an additional bit of CO2 to be injected because of surface turbulence.

And thanks for the insights and information Steve. I've always wondered if CO2 levels "in the wild" could be anywhere near where we keep them in our tanks and why ~30 ppm CO2 seems to be required for some plants in an aquarium while they're not likely to see levels anywhere near that in their native environments (please correct me if I'm wrong here).

 

[RoLeX]

OMG i have the best idea, it may have been thought of before or i could be wrong,

ok heres my idea, since we dont want surface agitation (to keep co2 in), but we still want sufficient levels of O2 in our tanks for the fish, y cant we run another reactor/diffuser for O2, simply have an air pump with a valve going at around 1 to 3 bubbles per second into the same diffuser or a seperate one.

This way we get the O2 into the tank in high numbers and it wont create surface agitation. Of course u would only have to run it at night when ur O2 levels drop and u can still retain a high level of co2 as well.

Will this work? and if it does do u think it is a good idea?

 

[malkore]

I imagine it would work. O2 from plant leaves dissolves into the water (unless it's saturated, then you have pearling instead) so I don't see why it wouldn't work.

I suspect with a good diffuser, you'd only need a few bubbles per minute to keep up O2 saturation. Realistically you shouldn't be below 6ppm, and if the saturation is 8.1ppm, that only leaves 2ppm needed, and I'd bet O2 dissolves about as readily as CO2.

A good idea...hard to say. But I know myself and Travis would both love to see the results of such an experiment, and I'd wager Steve would too.

 

[Zezmo]

The plain air diffusor sounds interesting, any reason you couldn't just put a second line, with air, into your existing reactor?

 

[malkore]

I'd just be afraid one standard reactor wouldn't be able to keep up with both gasses mixing/dissolving in the same chamber. As long as the reactor was beefy enough I think it'd be fine...or if you had a low Kh, and low turbulence, so that you had a pretty low bpm for CO2 flow, I bet you could stick your O2 line in at that point.

maybe i'm just over analyzing it... a single reactor would probably work fine.

 

[Zezmo]

Well, the reason I am thinking of this, I recently changed from HOB style filtering to a Canister, so now my tank has almost zero surface agitation. Both intake and outflow are submerged, with the outflow also being my reactor. My CO2 (as mentioned in another post) PPM has raised quite a bit. Well all week my rainbows have bee "funny" in the morning, not gasping..but sitting near the surface, very still... and missing the morning feeding. Day before yesterday, i was trying to see what was up..and my Iran Red got startled...he jumped out of the water, smashed into the lid... swam and crashed into both ends of the tank... he shattered his face, and died the next day ;(.

All my params are where they should be, and I have a full set of tests... I have been searching for some reasons... he died from being startled and injuring himself... but something was making the Rainbows "groggy" in the morning. someone mention acidic water from high CO2 at night... i will do some midnight testing. But a simple second line on a timer, with air into my existing reactor. Well thats easy to do, and may be just enough to keep things "safe" at night.