Quick take
Check whether the odor appears in cold water, hot water or both. That single test narrows the diagnosis dramatically before you buy filtration.
Cold and hot water both smell
If the odor occurs throughout the home in cold and hot water, the source is more likely in the well or incoming water. Hydrogen sulfide can form naturally in groundwater.
Only hot water smells
If cold water is normal but hot water smells, the water heater can be the source. Reactions involving the anode rod and sulfur-reducing bacteria are common suspects.
Why carbon alone can be the wrong fix
Low-level odor may improve with carbon, but sustained hydrogen sulfide often needs oxidation and backwashing media. High iron or manganese may need to be handled in the same treatment train.