kH, CO2 and pH? That old chestnut

[Clare]

Hello All….questions re kH/pH and CO2 from a newb!

Some background - My 9 month old tank’s kH seems to have drifted to zero (from around 2). I’ve been advised to increase to 4 for the sake of the livestock (I’ve lost a fish recently). Injecting CO2 with pH ranging from 7.4 to 6.1 (I reckon this swing is too much?)…

I’ve be pondering the CO2+H2O reaction:

CO2 + H2O <~> H2CO3 <~> HCO3- + H+

…and the role of adding carbonates.

I’m familiar with the role of enzymes to control the rate of these reactions (carbonic anhydrase), but don’t know much about it in the aquatic environment where these enzymes are absent.

My questions are:
- is it correct that dissolved CO2 is the only form of carbon used by plants (rather than H2CO3 or HCO3-?) Ie. When we inject CO2, it is only the dissolved form of CO2 that is useful.
- if only dissolved CO2 is useful, I guess we are wanting to drive the reaction back to the left? Is this achieved by adding more carbonates?
- what percentage of CO2 remains dissolved as CO2 vs converted to H2CO3 and then HCO3-?

- by adding carbonates (to increase kH) what are we doing to this equation, and more importantly, to the available dissolved CO2 for plants?

I really look forward to your thoughts on this!

 

[Dennis Wong]

The majority of dissolved CO2 in water is aqueous CO2 and only a smaller percentage becomes carbonic acid [read the wiki page on carbonic acid for the explanation]. Plants can use both CO2 gas directly (in the form of CO2 mist) in addition to dissolved CO2 and carbonic acid. Some species from hardwater biomes can intake bicarbonates, but it is more energy intensive, most plants will only do this when aqueous CO2 is inadequate.

The solubility of free CO2 is actually more when KH is low, not high. So the idea that adding more KH will increase the amount of dissolved CO2 is wrong.

The only reason to adjust KH upwards is if you have livestock that prefers a higher KH or pH range. There is small subset of plants that grow better with moderate/higher KH values (Rotala sunset, Downoi, some vals), while the other 99.9% grow perfect with low/zero KH.

Your pH swing/range is typical for many planted tanks that run 0 or near zero KH. that, by itself isn't a big impact factor for most livestock. Changes in pH due to CO2 do not have the same impact as changes of pH due to KH on livestock.

You can see some examples of planted tanks running at 0 or near zero KH on this page:

gh-tds /is-low-ph-in-tanks-due-to-aquasoils-softwater-a-concern" data-host="www.2hraquarist.com" data-pending="false">

Are bacteria affected due to low pH/KH in aquasoil tanks?

Does the low pH caused by CO2 injection or aquasoil affect livestock or bacteria in the planted aquarium? Should I use buffers to raise the pH?

www.2hraquarist.com

 

[GreggZ]

My 9 month old tank’s kH seems to have drifted to zero (from around 2). I’ve been advised to increase to 4 for the sake of the livestock (I’ve lost a fish recently). Injecting CO2 with pH ranging from 7.4 to 6.1 (I reckon this swing is too much?)…

As Dennis said above the vast majority of fish will do well in very low or zero dKH water. If you lost a fish it's very unlikely that it is due to the pH/dKH level.

And the drop from 7.4 to 6.1 is not too much. In my planted tank full of Rainbowfish my daily drop is from 6.25 to 4.85, a 1.4 pH drop. Plants and fish are thriving. And I know many other successful people in the hobby who run their tank in a very similar fashion.

 

[Clare]

Thanks so much for your replies.

Two topics here I guess:

1. Fish health in relation to kH and pH:

- Re kH - I find my visits to the LFS so incredibly frustrating. They were absolutely adamant that v low kH was the cause of the problem with my fish (one dead coryadoras, another with a small white lesion on it’s lip). (They are primarily marine specialists, and their freshwater/planted tank advice doesn’t fill me with confidence).
- considering I didn’t have any other answers (I was thinking columnaris v fungal infection, but the LFS didn’t even know how to spell it when she was “googling” it!)…..I thought raising the kH a little wouldn’t hurt anyway, and it makes sense that osmoregulation may be an issue with kH of zero…?

- pH - swinging pH and it’s effect on fish seems to be a real point of contention. Great to hear that your rainbows cope well with such a swing Greg.
….something I don’t understand is the frequent comment that pH swing caused by CO2 doesn’t effect fish, but a pH swing from other causes does. Isn’t a hydrogen ion a hydrogen ion, no matter what molecule it dissociated from??
I’m wondering if what people are really saying is that pH (other than extreme ranges) doesn’t cause a problem with fish, period. What does cause a problem, is whatever the “non-CO2” cause of the pH drop is Ie. the pH change is the “symptom” of an underlying problem in the tank.

Is this correct?

2. CO2/H2CO3/HCO3:

Musing about this is what kicked off my thread. As I was adding “alkaline buffer” to my tank I was trying to understand what I was really doing to this equation…
…..and if there are ways to increase the bioavailability of CO2 to the plants.

I couldn’t find any actual numbers about this online. Do any of you have a resource for this? Eg. %CO2 dissolved vs bound to H2O as H2CO3, and rate of dissociation to HCO3 and H+?

I’ve read only about 1% of dissolved CO2 becomes H2CO3, and if plants can actually use H2CO3 anyway, I guess it’s all a mute point!

Thanks again

 

[*Ci*]

I’m definitely following this thread, as I have similar issues (unexplained fish mortality) and questions. With my background in koi ponds, I’ve always understood that stable pH is a very important factor in fish health and routinely buffer my ponds against fluctuations due to rain, water changes, algae growth and large biofilters consuming kH in order to function.

I’m having to re-learn (and un-learn) concepts regarding this topic, and feel like I am still floundering a bit in the dark : )

 

[Dennis Wong]

You can read Tom barr's very old reply on the topic of why pH swings from CO2 are different from pH swings due to KH changes:

pH swings from C02 - Danger to fish?

I have a 72 gallon bow front that is pretty heavily planted, stocked with various tetras and amano shrimp, C02 injection, EI dosing for the ferts, and about 3.5 WPG light. The tank is running along great with the exception of small amounts of BBA, which I have been combating with excel spot...

barrreport.com

You can also run simple experiments with your culls/unwanted fish. I ran these experiments years ago, but did not publish the results as folks generally do not want to hear about pet fish being harmed intentionally for testing purposes. [even though literally every item for human consumption goes through similar tests. - Have different containers with a single parameter different from your holding container for the fish, then move them around and take notes on lethality rates etc. Its a simple science experiment.



I tested about 12 species of common aquarium fish (tetras, corydoras, barbs, white cloud minnows) and 2 species of shrimp across a few days.
A few interesting general observations: if you have generally healthy fish stock to start with, its very hard to kill them with just a single parameter shift. The parameter shifts I tested; pH, GH, TDS , KH, ammonia, Chlorine, Salt (NaCL)

Things that could kill fish:
Chlorinated water; 2ppm of chlorine resulted in 100% death rate across the board within an hour.
6+ dKH changes (with corresponding pH changes) in a short window - a few deaths, but not 100%, some deaths took hours to days to occur, so its not instantaneous.
1600 TDS change - small percentage of deaths (surprisingly), took hours to play out. Moving fish from 100 TDS water to 1600+ TDS water resulted in observable peeling off of the slime coat. However, most fish tested did not die after 24 hours of observation, it took longer than that.

Things that had zero deaths:
Non-KH pH changes; such as increasing pH from 5.0 to 7.0 instantaneously by moving fish from acidic, CO2 saturated water to neutral non enriched CO2 water.
GH/TDS changes within the ranges I tested. A 10dGH swing either direction seemed to do nothing at all, similar for 500 TDS swings. After a 3 day window, fish were feeding normally with no signs of distress.

I also tried 1ppm of ammonia in pH 7.5. Ammonia toxicity for hardy common species seem to take days to play out, so the 1ppm of Ammonia in pH 7.5 I tried couldn't induce deaths in the shrimps I tested on. However, it may lead to shortened life span or may take more days to play out.

Some general conclusions, with observations from having kept sensitive fish as well:
In the above tests, other than chlorine, every other factor tested did not result in quick death (within the hour). Stresses due to changes can take hours or days to play out. So for folks wanting to do such testing, fish health should be evaluated across a longer time horizon. Large pH swings due to the replacement of CO2 enriched with non CO2 enriched water has no impact. If this was not so, every CO2 injected tank would be losing various form of livestock every time we water changed when the tank water was CO2 saturated.

Small changes in GH has no impact. However, while both the pH change and GH changes have no impact, livestock may well have their own preferred ranges for parameters. So they will react to if they are brought out of their comfort zone. For example, if you have fish that prefer acidic water; having a pH swing from 5.5 to 6.5 will probably have no impact on them, but changing the pH from 6.5 to 7.5 may induce a change. Similarly, if you have shrimps that prefer higher GH ranges, like neocardina species, changing the GH from say 9dGH to 17dGH will have no impact on the shrimp, however, dropping it from 9dGH to 1dGH, you will likely get some deaths. Unfortunately for all parameter tests, it can be difficult to separate a relative change from the absolute values.

"when the only tool you have is a hammer, everything becomes a nail"
If the only tool I give a hobbyist is a pH meter, the poor guy will try to correlate every change in his tank to pH changes. Same as if I gave the guy a nitrate test kit only. Hobbyists are extremely blinded by the few tools available to them, and are largely tunnel visioned into over-interpreting results that occur due to much larger range of variables.

While hobbyists over analyze data that is easily quantifiable by test kits and modern equipment, there are a ton of other environmental factors that can affect fish health; from the microfauna/microbial mix in each tank to factors such as stress from tank mates or lighting systems or patterns of flow/lack of hiding places etc. Fish stock itself can be variable from source as well.

Be wary of folks that have only tried one method of doing things

 

[BryceM]

My background is in engineering (by training) and medicine (currently a practicing surgeon). I've been keeping fish for 40 years and have bred and sold hundreds of fish. In my opinion, the majority of fish deaths stem from disease. At times I've kept 50+ fish alive and healthy for over a year in a 180g tank without a single death. At other times, I've lost 50% of the fish within a couple weeks. During this time, the tank was routinely cared for and the fert/feeding/water change regimen was steady. What changed? The introduction of a few new fish.

Bottom line. Many of the fish we buy aren't very healthy to start with. They're raised in conditions to maximize productivity and sales. Shipping is rough, and the care they get at the LFS is sometimes pretty dismal. Disease can easily spread and you can try all the antibiotics/antifungals/antiparasitics in the world. In my experience, there's little you can do to change the outcome. You probably did nothing wrong and you can hardly expect the "experts" at a local fish shop to know anything about why fish are dying. As has been eloquently stated above, the KH probably had nothing to do with it.

The original question about KH/pH/CO2 is an interesting one. After everything that's been said about it, I'm of the opinion that the best way to figure out CO2 concentrations is to add enough CO2 to cause a 1.0-1.2 pH drop. We all discuss this like CO2 is the only source of acid in our aquariums. After keeping my current setup running non-stop for 5+ years (no change in substrate), there's very clearly more acid around than CO2 can account for. I assume it comes from decomposing plant material, bacteria, and the fish themselves (organic sources).

 

[Clare]

Thanks very much for your reply, it’s appreciated. (Sounds like we have similar education and professional experience, but vastly different fish keeping experience!!)
I’m only new to this aquarium business (and quite obsessed!) and instinctively attempt to apply my knowledge from other fields to my aquarium, which is why I find the replies in this forum so helpful and interesting!

Your thoughts related to diseased fish is interesting. How long do you advise quarantining newly bought fish?

It’s been more than 8 months since I added a new fish to my tank. (A school of ember tetras).

I hadn’t noticed any signs of illness with the Cory so was quite shocked when I found it deck. There are macrobrachiums in the tank, so there is the remote possibility it was injured by one of them?

Unfortunately I’ve lost another fish this week. A 6/12 old Columbian tetra which was bred (as a surprise!) in this tank. The only change in parameters has been raising the kH from 0 to 3 in 4 days. (Perhaps too rapid a change?)
…It developed what looked like a swim bladder issue. I was all for nuking the tank with antibiotics but was advised by LFS to give it salt baths which I did. Unfortunately it eventually succumbed.

Not sure where to go from here.

Just watch and wait, I suspect, keeping a close eye on the white lesion on the Cory’s lip.

 

[Clare]

You can read Tom barr's very old reply on the topic of why pH swings from CO2 are different from pH swings due to KH changes:

pH swings from C02 - Danger to fish?

I have a 72 gallon bow front that is pretty heavily planted, stocked with various tetras and amano shrimp, C02 injection, EI dosing for the ferts, and about 3.5 WPG light. The tank is running along great with the exception of small amounts of BBA, which I have been combating with excel spot...

barrreport.com

You can also run simple experiments with your culls/unwanted fish. I ran these experiments years ago, but did not publish the results as folks generally do not want to hear about pet fish being harmed intentionally for testing purposes. [even though literally every item for human consumption goes through similar tests. - Have different containers with a single parameter different from your holding container for the fish, then move them around and take notes on lethality rates etc. Its a simple science experiment.

View attachment 2240

I tested about 12 species of common aquarium fish (tetras, corydoras, barbs, white cloud minnows) and 2 species of shrimp across a few days.
A few interesting general observations: if you have generally healthy fish stock to start with, its very hard to kill them with just a single parameter shift. The parameter shifts I tested; pH, GH, TDS, KH, ammonia, Chlorine, Salt (NaCL)

Things that could kill fish:
Chlorinated water; 2ppm of chlorine resulted in 100% death rate across the board within an hour.
6+ dKH changes (with corresponding pH changes) in a short window - a few deaths, but not 100%, some deaths took hours to days to occur, so its not instantaneous.
1600 TDS change - small percentage of deaths (surprisingly), took hours to play out. Moving fish from 100 TDS water to 1600+ TDS water resulted in observable peeling off of the slime coat. However, most fish tested did not die after 24 hours of observation, it took longer than that.

Things that had zero deaths:
Non-KH pH changes; such as increasing pH from 5.0 to 7.0 instantaneously by moving fish from acidic, CO2 saturated water to neutral non enriched CO2 water.
GH/TDS changes within the ranges I tested. A 10dGH swing either direction seemed to do nothing at all, similar for 500 TDS swings. After a 3 day window, fish were feeding normally with no signs of distress.

I also tried 1ppm of ammonia in pH 7.5. Ammonia toxicity for hardy common species seem to take days to play out, so the 1ppm of Ammonia in pH 7.5 I tried couldn't induce deaths in the shrimps I tested on. However, it may lead to shortened life span or may take more days to play out.

Some general conclusions, with observations from having kept sensitive fish as well:
In the above tests, other than chlorine, every other factor tested did not result in quick death (within the hour). Stresses due to changes can take hours or days to play out. So for folks wanting to do such testing, fish health should be evaluated across a longer time horizon. Large pH swings due to the replacement of CO2 enriched with non CO2 enriched water has no impact. If this was not so, every CO2 injected tank would be losing various form of livestock every time we water changed when the tank water was CO2 saturated.

Small changes in GH has no impact. However, while both the pH change and GH changes have no impact, livestock may well have their own preferred ranges for parameters. So they will react to if they are brought out of their comfort zone. For example, if you have fish that prefer acidic water; having a pH swing from 5.5 to 6.5 will probably have no impact on them, but changing the pH from 6.5 to 7.5 may induce a change. Similarly, if you have shrimps that prefer higher GH ranges, like neocardina species, changing the GH from say 9dGH to 17dGH will have no impact on the shrimp, however, dropping it from 9dGH to 1dGH, you will likely get some deaths. Unfortunately for all parameter tests, it can be difficult to separate a relative change from the absolute values.

"when the only tool you have is a hammer, everything becomes a nail"
If the only tool I give a hobbyist is a pH meter, the poor guy will try to correlate every change in his tank to pH changes. Same as if I gave the guy a nitrate test kit only. Hobbyists are extremely blinded by the few tools available to them, and are largely tunnel visioned into over-interpreting results that occur due to much larger range of variables.

While hobbyists over analyze data that is easily quantifiable by test kits and modern equipment, there are a ton of other environmental factors that can affect fish health; from the microfauna/microbial mix in each tank to factors such as stress from tank mates or lighting systems or patterns of flow/lack of hiding places etc. Fish stock itself can be variable from source as well.

Be wary of folks that have only tried one method of doing things

Very interesting stuff! What a great approach to working through the impact of these variables!

I suppose in my setting, I’m worrying about the long term issues on the fish, rather than over a few hours. I confess I’ll need to read over your comment a couple of times, to mull it over.

I totally agree with your comment re hammers and nails. My collection of various hammers has grown steadily as I do like to try to measure and control as much as I can, which I’ve been told is a typical trait of beginners. It must be a complex closed system we have going on in these glass boxes!

At the end of the day, having plants not quite as amazing as they could be, or clumps of hair algae driving you crazy is bad enough, but it’s much worse when livestock die. As their “custodian” I want to make sure there isn’t anything I can do to improve their environment.

Thanks again. I’m sure I’ll have more questions to follow!

 

[BryceM]

I would really try to avoid adding antibiotics to your main aquarium. If I ever do it, which is pretty rarely, I do it in a hospital tank.

To answer your question about quarantining new fish - it depends on the stakes. If they're going in with a dozen discus, I'd probably quarantine for a couple months - but that's an extreme. Most nonsense will show up in a week or two. Right now I have 20 new cardinal tetras in quarantine. A couple of them have developed some swim bladder issues..... so they're still there.

As far as the frustrations of keeping a planted tank..... they're real. When it all comes together it's fun though. Right now I have a 180g heavily planted tank and I've got 3 or 4 separate spawns of kribs, two pair of routinely breeding angelfish (of 6 total), and a bunch of other fish that seem to be pretty happy at the moment. 2 rummynose in the tank are more than 5 years old! Algae issues were driving me nuts for a while, but after a long period of neglect (years) I'm more or less getting it dialed in again in terms of light, ferts, CO2, and everything else. I killed over 100 fish once when a CO2 solenoid got stuck open. Stuff like that is a real gut punch. You just do the best you can. Most of these fish have an amazingly high mortality rate and short lifespan in the wild, so they're better off in our care most of the time.

When it comes to cories.... don't ask me. I've had pretty poor luck with them. Some in my care have lived for 5+ years, but most of the time they don't last more than a few months. High CO2 seems to stress them out more than other fish. I don't know if it's that, or something else.

 

[Dennis Wong]

Thanks very much for your reply, it’s appreciated. (Sounds like we have similar education and professional experience, but vastly different fish keeping experience!!)
I’m only new to this aquarium business (and quite obsessed!) and instinctively attempt to apply my knowledge from other fields to my aquarium, which is why I find the replies in this forum so helpful and interesting!

Your thoughts related to diseased fish is interesting. How long do you advise quarantining newly bought fish?

It’s been more than 8 months since I added a new fish to my tank. (A school of ember tetras).

I hadn’t noticed any signs of illness with the Cory so was quite shocked when I found it deck. There are macrobrachiums in the tank, so there is the remote possibility it was injured by one of them?

Unfortunately I’ve lost another fish this week. A 6/12 old Columbian tetra which was bred (as a surprise!) in this tank. The only change in parameters has been raising the kH from 0 to 3 in 4 days. (Perhaps too rapid a change?)
…It developed what looked like a swim bladder issue. I was all for nuking the tank with antibiotics but was advised by LFS to give it salt baths which I did. Unfortunately it eventually succumbed.

Not sure where to go from here.

Just watch and wait, I suspect, keeping a close eye on the white lesion on the Cory’s lip.

just a quick reply - 0 to 3dKH change in 4 days isn't much for common livestock species. However, if a specific species of livestock require low KH water (0-1 kh?), such as some sensitive Caridina shrimps, or fish that need very softwater, then that change will have significant impact.

If you want to see a large data set of tanks that always have changes (gradual) in KH, look at aquascaped tanks that utilize Seiryu rock. The rock constantly leeches carbonates, so the tank's KH continually rises. Usually the user would try to offset this with regular water changes. In my tank below for example; the KH is around 4 after a large water change (50-70%), it rises to 9 to 10dKH within the week, then drops back to 4 when I do the water change. So in this tank, and every other tank that uses a significant amount of Seiryu rock, the KH is never constant. Common aquarium fish and shrimps are fine - I had more than 1 species of fish that breed in this tank and the amount of shrimp was endless.