Ah yes the old Hydrogen sulfide from bacteria. I had a touch of it and couldn't figure out why. I had removed the poly tank and bleached it, replaced the lines and after a season started to have it again. I realized that the tap water that I was adding was from the kitchen sink has a carbon filter that absorbes all of the clorine in the water. SO there was a lack of treatment for bacteria. Bleach (hypochlorite) works well. Another treatment that can be used is Hydrogen peroxide, but do not mix the two!
Hydrogen peroxide sanitizes without the bleach stench but is more expensive to use. Once under control, sanitize at least once a year.
Cal 9.2 #19 SilverSwan