Hydrogen sulfide is dangerous even to people. It comes from decaying waste and organic material. Stirring your sandbed could be lethal to your tank if the levels get high enough. You need to thoroughly siphon your sand bed. Your sandbed is a nutrient sink, it is becoming a toilet. Either clean the sand very thoroughly or replace it all together before the problem becomes unfixable. Just my advise. Many people disturb the sand when this problem occurs and massive death ensues. Releasing all the gas at once is not a good idea generally. You need to keep your sand clean.
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.