I always make sure I purchase Zeobak that came in directly from the supplier as retailers don't usually refrigerate it. Doubt suppliers do as well but the fresher the better I figure. The expiry dates on the bottles seem to disagree with what's written in the guide.
From the guide:
Quote:
This product has a limited shelf life because it contains cultures of living microorganisms.
Unopened, it will last for at least 6 months if kept refrigerated. After the bottle is
opened, the shelf life is approximately 3 months. Spoiled strains can be identified by a
foul odor. If this is the case, some of the microorganisms are dead and the solution
cannot be used. We recommend purchasing a bottle size that can be used in a 3-month
period. It is better to purchase smaller amounts on a more frequent basis. Please make
sure to avoid any contamination of the solution in the bottle (e.g. with aquarium water).
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Yet the last bottle I purchased has an expiry date of 2015.
I wish I had a microscope so I test for certain but for now I rely on the odor test.