Category Archives: Gases & Exchange

Hydrogen Sulfide Pond Gas – Not Very Common

Hydrogen Sulfide gas is a gas produced in an-aerobic pockets under water. These may occur anywhere there is deep sand or sediment, or in a sump with thick mulm.

Hydrogen Sulfide Pond Gas – Not Very Common

Without Oxygen, some bacteria can respire by using Sufur, and this creates Hydrogen Sulfide, H2S which is very, very toxic to fish.
HACH Chemical has a test for this,

Hach Company
P.O. Box 389
Loveland, Colorado
Phone: 800-227-4224
Fax: 970-669-2932

…but H2S intoxication should be suspected in any case of fish loss where:
1) There are no parasites, proven by microscopy,
2) There are no real Ammonia or Nitrite derangements
3) The signs are respiratory and
4)The history has some reference to a stalled filter or the stirring of stagnant gravel or media.
5) NEUROLOGICAL signs, of spinning, flipping and fearlessness may be seen just before death occurs.

The smell is one of rotting eggs. Losses may be great and they will continue after the H2S is long gone. If you discover or suspect H2S poisoning, you should REMOVE THE FISH before disturbing ANY gravel or sand.

Hydrogen Sulfide Pond Gas - Not Very Common
Hydrogen Sulfide Pond Gas – Not Very Common

Also, to neutralize SOME of the hydrogen sulfide, a weak dose of Potassium permanganate may be effective. Still, housekeeping, and finding the source of the anaerobic bacteria is the priority.

I’ve heard from a LOT of people that Aquascape ponds have Hydrogen Sulfide problems. I have tested dozens of these, especially the older ones since they are the most lored to have a problem – and have found NO H2S. My kit is the pad test from Hach. H2S is not an issue of pond TYPE but purely of pond maintenance.

Carbon Dioxide in Aquaria and Ponds

Carbon dioxide is produced by the respiration (not photosynthesis) of both plants and animals. When you exhale, you produce Carbon Dioxide. Pretty much the same with fish except they don’t have bad breath like you do. Just kidding.

One interesting thing about water is that Carbon Dioxide levels can exist independently from the Oxygen concentration. For example, you can have water with plenty of Oxygen, but which also has a lot of Carbon Dioxide.

Carbon Dioxide in Aquaria and Ponds

Carbon dioxide has a partial pressure in the air, when oxygen goes up, carbon dioxide has to go down. In water; NOT SO. You can have crazy CO2 levels with high O2 levels and vice versa or any mix.

Carbon Dioxide in Aquaria and Ponds
Carbon Dioxide in Aquaria and Ponds

Carbon dioxide likes to do one thing in water. It likes to convert to carbonic acid. When it does this, it tends to affect pH by bringing it down into the acid range. So, a bunch of fish breathing in a small tank with minimal circulation and surface agitation may actually accumulate sufficient carbon dioxide to drag down the pH due to the carbonic acid equilibrium that will result.

Removal of carbon dioxide (with its carbonic acid behavior) by increasing surface exposure and gas exchange will remove the carbonic acid and may raise the pH above neutral.

Do you remember in the discussion of Ammonia when I mentioned that Ammonia is more toxic at a higher pH? Well, this created a problem for some fish once. Someone had the idea that if you deprive the fish of aeration, you will preserve a high carbon dioxide level. The carbon dioxide level will keep a higher carbonic acid level, which will keep the pH down. The lower pH will ionize the Ammonia to Ammonium and preserve the fish from Ammonia damage at shows.

So, at least for a while, it was a widely perpetrated myth to enjoy gasping fish at shows, and use autogenous Carbonic acid to lower the pH, rather than do a water change to remove the threat from Ammonia.

The most amazing myth: “Don’t oxygenate Show Tanks” because it blows off carbon dioxide [carbonic acid], raising the pH and causing the ammonia to become more toxic.

Hydrogen Peroxide For Dissolved Oxygen

Hydrogen Peroxide For Oxygenation
– by Dr. Erik Johnson

Hydrogen peroxide can be used to increase the dissolved oxygen in the system. The peroxide molecule dissociates into water and an oxygen free radical which instantly bonds into 02.

H202 + H20 ———-> H20 + 02

hydrogen peroxide koi
hydrogen peroxide koi

Simply use 1/2 cup per hundred gallons. I prefer, however, to apply peroxide in a typical spray bottle, squirting it forcefully under the surface of the water into tanks and vats in trouble, I use 60 squirts per 100 gallons.

Hydrogen Peroxide For Dissolved Oxygen

What you will see is the fish will leave the surface, and swim about as if there was no dissolved oxygen deficit. The time for this to occur is approximately one hour. The benefits are seen for a full 4 hours.
The hydrogen peroxide to use is drugstore 3% peroxide.
It’s great in emergencies.
If applied directly to the fish, or applied directly to their water in an undiluted form, gill damage could result, so apply it away from the fish.

What Should I Know About Feeding Koi in Warm Water?

Feeding fish in warm water is an interesting conundrum. To succeed you need a strong equilibrium between the fish, the water’s oxygen levels and the demand for it by beneficial bacteria (Equilibrium by luck or by design). The fish NEED a lot of food because they are burning a LOT of calories. The pond’s biological reduction system is optimized and working ferociously on fish wastes. Warmer water carries less oxygen. Still, in most ponds a natural balance is forged which can usually handle this. But there’s risk.

Feeding Koi a lot of food in the warm months is desirable, and it ensures good health and growth. But if you go too far, and overfeed them, the water quality will deteriorate and if you overfeed enough, there can be a sudden bloom of bacteria that will:

1. Cloud the water

2. Weaken or stress the fish

3. Consume much, if not all, of the available dissolved oxygen.

Feeding Koi, or Overfeeding Koi?
Feeding Koi, or Overfeeding Koi?

Don’t take this lightly, because even with your years of success, you can get into trouble. I did this one summer. I fed heavily and the fish were doing well. What I did not acknowledge was that my oxygen levels were TEETERING in the danger zone because of the ferocious use of oxygen by the fish, the feeding and the biological bacteria, and the warmth of the water. I went to net my favorite fish up, and she simply stroked out for lack of Oxygen in her peak metabolic condition and then compounded by warm water and the chase.


What Can You Do?

Well, waterfalls do a LOT to contribute oxygen to the scenario. So breathe a sigh of relief if you have a robust set of falls. Additional water pumping with a spray bar can increase oxygen. If the pond is in filtered sunlight or only gets baked for part of the day, it will be cooler, and contain more oxygen. Shade cloth is employed seasonally on some ponds in the hot Southwest. “Feeding hard”, but being alert is an ingredient for success. And finally, intermittent water changes can simultaneously contribute to a coler ambient temperature and reduce the fuels used by damaging bacterial blooms.

Water Change Routine During Feeding Season:

10% per week

20% every 2 weeks

30% every 3 weeks

I am a lazy Koi keeper, so I run water to my ponds all day every day via a drip irrigation system which can replace between 12 and 100 gallons a day. I never drain and fill water change anymore.

Aeration – Oxygenation of Pond Water

Aeration – Oxygenation of Pond Water

Aeration – by Doc Johnson
aeration in ponds“Warm water carries less dissolved oxygen than cold water. Aeration becomes of paramount importance in the summertime. In the winter, low dissolved oxygen would be just about impossible. Trasnporting fish is dangerous in warm weather. The use of some ice packs in the water tank with the fish may reduce losses.” Doc Johnson
Aeration or “oxygenation” is perhaps one of the most important factors governing your success in the pond with your fish.
Inadequate aeration can cause deterioarted filtration performance, because the bacteria consume prodigious amounts of oxygen in the reduction of Ammonia.

Ammonia reduction
NH3 (Ammonia) ——> addition of oxygen ——> NO2 (Nitrite)

Also, fish require a sufficient amount of oxygen as well. Especially when being medicated, or when their gills are under attack by parasites or “bad” water quality.

One way to increase aeration is to employ a large airstone. More effective, is the use of a “venturi”-equipped pump-return because aerated water is returned over a larger area, and even more effective, a water fall or other water feature which disrupts the surface of the pond alot. I usually advise folks, if you can hear your pond splashing and babbling from ten feet, there’s probably enough oxygen. (At least within the eight or so feet of the feature).

Signs of inadequate aeration include gasping at the surface, or if less severe, the fish do not play – they just act lethargic.

Aeration – Oxygenation of Pond Water

There’s alot more in the book on dissolved oxygen, how to test it, and what medicines consume D.O. – – Even how much dissolved oxygen they consume. Another section describes a compound (hydrogen peroxide) which actually ADDS oxygen to the water in emergencies and so the book might be just what you’re after.
Aeration is under estimated in importance. You may have “adequate” oxygen levels but not necessarily optimal levels.

Aeration by flooming. A simple water pump aimed up at the surface.

There is probably an article on “flooming” in this web site. It’s the process of putting a little pump on the bottom of the pond and aiming it at the surface to create a slight swell of rising water on the surface of the pond. This is critically important in very deep ponds because oxygen penetration to the bottom of deep ponds is very challenged.
Dr Erik Johnson

Oxygen, Aeration, and Oxygenation in Ponds and Aquaria

Oxygen is essential to almost all living things. Water carries a pitifully small amount of oxygen but fish have been designed to make the best of it.

Oxygenation is atmospheric. Not bubbles.

The only way oxygen can mix with water (oxygenation) is by contact with atmospheric air in features such as waterfalls, flooming pumps, filter returns through spray bars, or air stones.
If you can “hear” your pond’s surface agitation, chances are there won’t be any problem with dissolved oxygenation unless the water is REALLY warm or you’ve used Formalin. (You can read about Formalin in the Medicine Cabinet chapter). A loud water fall is the surest sign that oxygenation issues have been addressed and averted.
If you do not have a water fall, make sure your filter returns agitate the surface, and / or employ a flooming pump. (See glossary).


Hey, it’s REALLY important that you know that warm water carries even less oxygen than water at any other temperature, and sometimes this causes problems in the summer. Large fish need much more oxygen than small fish, which is why large fish die first when the oxygen supply to the pond is discontinued. In the summer, when someone’s largest fish die, especially at night = think low oxygen levels.
You may also find it reassuring that in very cold water, oxygen levels tend to stay near their highest levels.

What this boils down to is that your fish are always going to require the circulation and aeration of their water, however you decide to do it.

Any way you choose, the objective is to disrupt the water’s surface and expose as much water to the surface as possible. This is why I prefer “flooming” to any other method. It’s easy and effective.

This is what I would recommend as far as dissolved oxygen.

  • First, make sure the pond always stays cooler than eighty degrees. This is not always possible in the Southwestern part of the country.
  • Secondly, always have two pumps moving water around the pond. Obviously, one of the pumps will push the filter. The second pump can drive a water fall or some other feature, or it can be set up to “floom”. Either way, if one pump chokes up and dies, the other pump can carry your fish until the failure is discovered.
  • Thirdly, make sure your two pumps are on entirely different circuit breakers in the fuse box. This way, if one pump shorts out, it won’t kick the breaker and take out the second pump. Running a second line from the fuse box is not much money, but it will save fish lives sooner or later. A power failure in the summer is death to fish in warm water.

That’s the whole point of this section on oxygenation levels and water quality.

Why Carbon Dioxide is a “Thing” in Fish Medicine and Their Fishy World

Carbon Dioxide Impact on Koi Fish Health via PH

Carbon dioxide is produced by most life forms in the process of respiration. When you (and the fish) “breathe out” you’re releasing Carbon Dioxide. Even plants respire, at night in the absence of light. They liberate carbon dioxide while they grow.

Carbon dioxide from the fish and plants will naturally accumulate in water, unless the water is adequately circulated. When water circulates and passes into direct contact with the air, the carbon dioxide is expelled in a process called “de-gassing”.

Unlike the way it is in the air, carbon dioxide and dissolved oxygen can exist in levels independent of each other.

For example, it is possible (while improbable) that you could have high carbon dioxide levels and also have high oxygen levels at the same time. You can’t do that in a room. It doesn’t happen in “air” because they have partial pressures.

When carbon dioxide levels are high in the water, the fish will naturally require that more oxygen to be present as their gills compete to absorb oxygen and expel their carbon dioxide.
Under most circumstances, carbon dioxide accumulations have little real significance to the hobbyist except in the way it impacts PH.

Carbon Dioxide Impact on Koi Fish Health via PH

The important thing to note is this: Carbon dioxide naturally forms carbonic acid in the water and this acidic compound is responsible for pulling the pH in an acid direction. (See pH discussion in water quality section)

SHORT AND SWEET: Carbon dioxide is made through the respiration of plants and animals and drags down the pH unless properly buffered.

Carbon Dioxide Impact on Koi Fish Health via PH
Carbon Dioxide Impact on Koi Fish Health via PH

Hydrogen Sulfide Gas is a Rare Problem But It Kills When It’s Present.

Hydrogen Sulfide is not a common problem in fish ponds, as far as we know. (It *is* a problem in fish tanks, though. Especially at a major cleanout and with disruption of the sand / fluorite etc.)

What this means to pond people though, is; pond people don’t test for Hydrogen Sulfide much, so we don’t really know whether it’s common or not.

Here’s how Hydrogen Sulfide is produced: A plant is set on the bottom of the pond. Or a huge statue. Fish wastes accumulate under the large and heavy object. Every time your spouse suggests you clean out the grossness under this large and unwieldy item, you come up with an excuse. Ooooh my back! So it doesn’t happen for a long time.

Anyway, after three years, a black stain appears in the center of the wastes trapped there, and in the black stuff, there are bubbles of gas. These are hydrogen sulfide bubbles. The smallest amount of hydrogen sulfide will start killing your fish. Especially if those bubbles are swept into the water column suddenly.

What’s happening in the stagnant media is that a bacteria exists in areas WITHOUT water movement and WITHOUT oxygen. In the opportune spaces, the bacteria make hydrogen sulfide gas.

As it accumulates in the water, the gas causes weakness, increased vulnerability to disease, and finally obvious symptoms of brain damage and death. Enough hydrogen sulfide could kill you in an enclosed space.

Clean Up

Clean up isn’t easy. If you suddenly decide to run the test for Hydrogen Sulfide and you discover that the huge statue is trapping it underneath, you can’t just go in and yank out the thing. Removing the object which is trapping the gas would liberate large amounts of hydrogen sulfide all at once; and all your fish would be dead before you could get them out of the pond.

You need to remove all the fish from the pond before the cleanup operation is begun.

SHORT AND SWEET: Hydrogen sulfide is a problem in stalled sand filters or ponds which contain layers of sediment on the bottom or in large and neglected planters.