The odor is from an anaerobic process that will produce hydrogen sulfide gas via the Sulfate pathway. Add MICROBE-LIFT/IND as both will work for you. Start with 1 to 1/2 gallon then add one quart for every one to two thousand gallons per week.
These cultures can block the anaerobic production of hydrogen sulfide. You may also wish to add Sodium bicarbonate to bring the pH up to about 6.9 to 7.