Rotten egg smell=hydrogen sulfide.
I think your best bet would be to ditch the old sand and start anew. Bacteria is dead now anyway, after the FW rinsing. If you absolutely do not want to get rid of the sand, then try rinsing it in a bit of chorine...then rinse, rinse, rinse it clean. Leave it out in the sun to dry, turning it over and over within a dedicated container.
Well, broomer, if you're interested in talkin' about stinky stuff, let me tell you....I mixed up a fresh batch of fish food in my little food processor a couple of days ago; apparently the dish washer did not clean out the processor's blade stem well enough and, while getting my 4th of July food ready, I smelled this disgusting aroma....yep, rotten food processor. Thank goodness I didn’t chop up any veggies for our meal in the processor or it would have been one stinky, rotten squid smelling 4th around the picnic table!