Well, VJM and Stone, you guys have inspired me to do some research on this mystery. Not from aquarium hobby sites, where myths do abound, but from scholarly articles, and, er, Wikipedia (because we all know that Wikipedia is never wrong, right?
). My faculty with scientific language is rather poor, however, so you might to do some digging on your own.
First off, hydrogen sulfide is toxic to fish. Perhaps this was never in question, but it's a good place to start. "Hydrogen sulfide inhibits oxidative phosphorylation by blocking the reoxydation of reduced cytochrome by molecular oxygen. The overall result is inhibition of energy metabolism by cells, similar to the effect of hypoxia.... Fish exposed to acutely lethal levels of hydrogen sulfide exhibit increased ventilation rates, followed by rapid cessation of ventilation, then death."
-Egg survival and fry development of northern pike limited by exposure to as little as .006 mg/L hydrogen sulfide (henceforth abbreviated as h-s because I'm lazy).
-Acute toxicity in bluegill eggs after 72 hours at .019 mg/L.
-Acute toxicity in bluegill fry after 96 hours at .013 mg/L.
-Acute toxicity in juvenile bluegill after 96 hours at .048 mg/L.
-Acute toxicity in adult bluegill after 96 hours at .045 mg/L.
-Chronic exposure to .002 mg/L for 826 days did not cause mortality in bluegill.
-Growth of adult bluegill retarded by exposure to .011 mg/L.
-Acute toxicity in channel catfish after 3 hours occurred with 1.4 mg/L exposure.
-Acute toxicity in channel catfish fingerlings after 3 hours occurred with 1.0 mg/L exposure.
-High concentrations of h-s responsible for poor growth in channel catfish in acidic waters. (What is a 'high concentration'? I don't know...
-The EPA "suggests a maximum [h-s] concentration of .002 mg...for fish and other aquatic life."
How is it produced
The production of hydrogen sulfide is part of the sulfur cycle. Anaerobic bacteria (bacteria that requires the absence of oxygen) decomposes organic sulfur, using it for energy. This process creates hydrogen sulfide as a waste product. Organic sulfur comes from animal/fish and plant residue. High rates of microbial activity lead to anaerobic conditions within mud.
The Big Question
So, what we really want to know is how much hydrogen sulfide is typically created in a closed environment (i.e., our home aquariums) and whether this amount is enough to cause toxic conditions in our fish. I'm not sure. Most scholarly studies and literature focus on ponds and lakes. If anyone knows of a scholarly
study that addresses hydrogen sulfide in home aquaria, I would love to see it. However, I have come across these facts. Check them out and decide for yourself.
-Photosynthetic bacteria can oxidize h-s to sulfur and sulfate in the presence of light and the absence of oxygen.
-H-s is oxidized by oxygen, creating first sulfur dioxide and then sulfate. This may occur readily in surface waters. Both waste products are "eventually" (wish I knew how long "eventually" was)
removed through absorption by plants and soil.
-Hydrogen sulfide is readily soluble in water.
-In water, h-s is a weak acid yielding bisulfide ions and sulfide ions. At a pH of 7, the ratio of bisulfide ions to sulfide ions is 1:1. As pH increases, the ratio of bisulfide ions to liquid hydrogen sulfide increases. Only above pH 12 will the sulfide ions be at a 50% concentration.
-Several species of soil, aquatic and marine microorganisms oxidize h-s to elemental sulfur.
-"Because it is a gas under ambient conditions, bioconcentration and food chain biomagnification are unlikely to occur." (This might be different in a closed environment?)
It would appear that there are conditions in our home aquaria that encourage the oxidization of h-s. But what would happen if a fish were directly hit by one of these bubbles before it has become soluble and/or oxidized?
Boyd, Claude E. Pond Aquaculture Water Quality Management
. Norwell, Massachusetts: Kluwer Academic Publishers, 1998. 145-147.
Toxicological Profile for Hydrogen Sulfide
. Washington DC: Agency for Toxic Substances and Disease Registry, nd. 115-118.
"Hydrogen Sulfide." Wikipedia.