[Psyire]

I thought this excerpt should be posted here:

When dissolved in water, the smell depends strongly on pH (which determines how much is in the volatile, hence "smellable," H2S form). Humans often can just detect hydrogen sulfide odors when the concentration is above about 0.029 ppb in freshwater. In seawater at pH 8.2, where only 6% of the sulfide present is in H2S, this odor threshold is likely higher, perhaps on the order of 20-fold higher (0.6 ppb). Fortunately, that threshold is below the lethal limit of many aquatic organisms (usually above 5 ppb; sometimes as high as 50,000 ppb), so odor often can be detected by humans before hydrogen sulfide rises to acute, lethal concentrations in reef aquaria.

Taken from here:

H2S & Reef Aquarium:
Randy Holmes Farley

http://www.reefkeeping.com/issues/2005-12/rhf/index.php