Duncan Griffiths Copyright FEBRUARY, 2003, D, GRIFFITHS, ALL RIGHTS RESERVED


Hydrogen Sulphide or H2S seems to strike fear in the hearts of most if not all fish keeping hobbyists, none more so than koi keepers, the author has often heard reports on the internet of H2S being responsible for mortalities in koi carp, however, although the author has seen its existence on many occasion in ponds all over the UK, he has NEVER seen any mortalities attributed to this strange gas, from that stand point H2S it appears it can be a problem but it seems to be a small one.

It must be noted the occurrence of hydrogen sulphide is not that unusual in lakes inhabited with carp and other species with or without extensive vegetation. H2Ss very existence is a fact of life, yet wild carp and indeed other species of fishes do not readily die because of its is presence.

Wherever raw sewerage is processed H2S is always present; although H2S is a sanitizing killer, bacterium essential for processing raw sewerage flourishes and the purified water is released into lakes and reservoirs, fish also abound even though it is present in some form or another.



Hydrogen Sulphide


Molecular formula H2S

Physical appearance colourless gas

Chemical family inorganic gas

Melting point -850c

Boiling point 600c

Water solubility 4370 ml/litre @ 00c; 1860 ml/litre @400c

Physical hazards flammable gas, may cause flash fire


Short-term inhalation exposure

Include irritation, lack of sense of smell, sensitivity to light, vomiting, breathing difficulties, lung congestion, nerve damage, heart damage, internal bleeding, brain-damage, coma and eventually death.


Major uses for H2S include dye manufacturing, tanning, cosmetics and wood pulp production; also therapeutic medicinal baths from natural spring waters containing H2S.



Hydrogen sulphide is quite common in the environment in low concentrations. E.g. mineral springs, bodies of water, waste water, and natural gases.

Hydrogen sulphide is manufactured during the decomposition sulphurous protein amino acids due to the influence of putrefactive sulphur anaerobic bacteria. E.g. standing water, marshes lakes and sewerage plants. Or, in the manufacture of chemical fibres, coking plants or refineries.


Hydrogen sulphide has an offending smell of "rotten eggs" which I am sure we have all smelt form time to time in our ponds especially if you have a water garden containing plants, the odour is detectable at very minute concentrations in the air. At higher concentrations (50-150 mg/m3it has a sweet sickly odour and at concentrations above this it can quickly deaden a humans sense of smell, leaving the subject unaware of its presence, therefore H2S in factories must be electronically monitored.

The threshold for smell and taste in water is estimated to be 0.05 0.1 mg/litre, the taste and odour threshold for sulphides is 0.2mg/litre



Hydrogen sulphide is formed when sulphides hydrolyse in water. Once H2S is a solute in water it quickly dissociates to form mono-hydrogen sulphide (1-)(HS-) and sulphide(S2-) ions. Methylene blue, colour-metric test kits are Ready available and quite cheap and will identify hydrogen sulphide at concentrations in the 0.1 and 20 mg/litre range


The concentrations of these two are dependent on pH of the body of water containing the hydrogen sulphide. The higher the pH the lower the concentrations of hydrogen sulphide and the higher the concentrations of dissociated mono-hydrogen sulphide. The lower the pH the higher the un-dissociated hydrogen sulphide.

At a pH of 7, 50.4% of the amount of sulphides will be hydrogen sulphide; the toxic form and the remaining 49.4% will be mono-hydrogen sulphide and sulphides.

At a pH of 5, 99% of sulphides present will be the toxic form of Hydrogen sulphide, at a pH of 9, less than 1.0% will be the toxic form of hydrogen sulphide.



1mg/m3 = 0.706 PPM

1 PPM = 1.416 mg/m3.




























0.02-0.4 mg /litre

Sub-lethal effects, gill damage, (dependent on species)

0.02-5.3 mg/litre

Lethal for some species

Generally the lower the organic load of the fishes natural habitat the more sensitive a species is, I.E. salmonoids are more susceptible than carp




Occurrences in the environment


Hydrogen sulphide is present in the atmosphere by natural emissions from concentrations of 0.1 1.0mg/m3, (mg=micrograms) although samples taken from near by factory emissions can elevate the concentrations locally, typically above 100 .mg/m3.


The estimated daily intake for a human averages10-20 .mg calculated on the assumption the 20 m3 of air containing hydrogen sulphide as calculated above, is inhaled.

Drinks and food may contain H2S, however the formation of sulphides while the cooking process takes place complicates available figures,

skimmed milk, less than 0.1% fat typically contains 0.8mg/litre while cream containing 30% fat and above can contain as much as 1.84mg/litre.

Cooked beef can contain 0.3mg/litre while lamb can contain 0.4mg/litre with levels being higher in anaerobically packed meats, with the average daily intake from food and drink per human being around 1.7mg/litre daily.

Water of course is a natural place to find H2S but they are most commonly derived from natural sources, I.E. underground sources such as well water from bore holes where water seeps up through anaerobic micro-organisms found in minerals rock formations.

In the USA, 500.mg has been found in fresh water supplies.


Hydrogen sulphide is easily absorbed in humans following ingestion; it rapidly spreads to the brain kidney, pancreas, liver and intestine.

Most consumed hydrogen sulphide is metabolised to form Methanethiol and Dimethyl sulphide most sulphides are quickly excreted via the kidney. Although a small portion of sulphide may be expelled by the lungs and some metallic forms in the faeces.



The LD50 for sodium sulphide given orally in rats was reported as 250mg/kg of body weight.

Water containing 4-12 mg /litre of hydrogen sulphide was found to be Embryo toxic in rats but water containing 2-3mg/litre had no effects at all, also no birth and growth defects were recorded either.
Human exposure to low concentrations of Hydrogen sulphide has caused corneal damage, headache, sleep disturbances, nausea, weight loss, and symptoms suggestive of brain damage.

Higher concentrations can cause irritation of the lungs even pulmonary oedema. Exposure around the 200 PPM has caused arm cramps, and low blood pressure, unconsciousness, after 20 minutes. Coma may occur within seconds inhaled at high concentrations and be followed rapidly by death. For example, workers exposed to 930PPM hydrogen sulphide for less than 1 minute died.
Hydrogen sulphide is moderately toxic to animals via the inhalation route: LC50 (mice, 1 hour) = 673 PPM; LC50 (mice, 7.5 hours) = 140 PPM. Exposure to 10-13 PPM for 4-7 hours has caused eye irritation.
Skin absorption of Hydrogen sulphide is slight and not considered significant. However, prolonged or repeated skin contact might cause mild irritation.


Treatment and control

The most effective way to deal with H2S is cleanliness failing that:-

To neutralise H2S is with chlorine to be fed in sufficient quantities to eliminate it, while leaving a residual amount in the water to prevent its reformation, (3 PPM of chlorine is required for each PPM of hydrogen sulphide).However chlorine is not of much use to the koi hobbyist,Anyone running ozone in their koi system will very quickly rip apart the H2S molecule KMnO4 dosed pond will also reduce Hydrogen sulphide levels quite significantly in the region of 70%But as good as both these methods are the title goes to good old H2O2.Trials showed that the application of hydrogen peroxide H2O2 not only eliminated 87-98% of high concentrations of hydrogen sulphide, but, because H2O2 dissociates in water to release extra O2 the effect of which is to enrich the oxygen saturation in the water the reformation is not so evident, and the residual effects of H2O2 working on H2S can be felt for some hours after the H2S first vanished. Activated Carbon filtration may further reduce H2S as will excess aeration.


So to recap!

There is probably more information than was required for this topic but fore-warned is fore-armed

From the koi hobby point of view it seems it can be both myth and monster, but a reality check is called for.

First of all hydrogen sulphide when compared to other koi disease and problems, is small indeed.

In a clean system H2S will not be a problem and further to this as the graph shows even if it was if you normally run high pH levels in the 8's if H2S was evident it would be at very low levels and would not pose much of a threat, further there are treatments available to the hobbyist to mop up this small proportion of hydrogen sulphide, namely H2O2 or KMnO4, in normal therapeutic doses






Hydrogen sulphide, Geneva, world health organization.

H2S control municipal water-water reference library

Material safety data sheets