I had great success with this stuff in my 10g tank so I bought more today, one bottle for myself and one for a gift. We were rushed doing last minute shopping and I just grabbed the only two bottles on the shelf. The sell by date is 12/2012. Should I be worried that it won't work? I'm not sure how long it will be before it goes in the tank. We live pretty far from the Petsmart that sells it so I'm kicking myself.