I don’t think prime actually expires.Well, this is Monsterfishkeepers so lots of people need huge tanks. I find the Prime works best for me since 1 capful treats 50 gallons which is the size of my container to treat water in. This bottle of Prime I got quite some time ago in a 2 for the price of 1 deal and I can't find a date on it so I hope it's not past its shelf life. Seems to still be working and smells just as atrocious in the bottle.
***someone may chime in to correct my thinking, I’m about 99% positive