It will improve water clarity, reduce ammonia and nitrogen levels, reduce build up of organic sediment, reduce noxious odors caused by fish urine, fish feed, fecal matter, algae, overfeeding, etc, reduces hydrogen sulfide odors and improves dissolved oxygen levels.
The smell is attributable to the natural production of hydrogen sulfide by sulfate reducing bacteria as part of the ecosystem. This provides a source of energy for the purple sulfur bacteria in the product and as a metabolic inhibitor, preserves the product, giving it its long shelf life while in the container.
If it is pinkish or brownish in color, it should be alright. It is definitely bad if it is whitish-grey and has a burnt rubber smell. The bacteria are definitely dead in that case.
No, we do not list expiration dates on the products. There is a batch number on each that can be traced when necessary. (See shelf-life for more information.)