[RichH]

Probability is that a distinct species of bacteria growing in the tank is giving up the sulphur (and H2S) when the dead cells decompose. Get inside the tank and scrub off all the bacterial slime from the walls and tank top, Shock sanitize the whole system with a heavy dose of Chlorox, replace all polymer componets (including vent hose) that have turned BLACK. When clean maintain tank hygiene by adding Chlorox to the amount that is "barely perceptable" with you nose at the outlet.

Take a large wad of sterile gauze and tape it over the end of the VENT hose to prevent reinfection of mold spores, etc. Change yearly and keep dry.