are you talking about the arag-alive stuff?
i don't think it can ever "go bad" per say.......it just won't be as effective to fight amonia/nitrites/nitrates. if it has expired only a couple of months ago, i would probably still buy it if you can convince the manager to give it a MAJOR markdown, like 50-75%. if they won't mark it down that much, tell them you'll pass and it can sit there on the shelf for the next 9months....lol.