Hi Josh,

I work with tetracycline a great deal in my research, and I have come across this phenomenon too - though I tend to dissolve my tet in methanol these days; it still happens eventually, probably due to to cumulative condensation forming on the inner surface of the freezing tube each time it's opened.

A quick look at the Sigma Aldrich technical data (note: this links directly to pdf) indicates that solutions [of tetracycline] in water become turbid on standing due to hydrolysis and precipitation of tetracycline.

According to another pharmaceutical information website, hydrolysis results at a pH value of 3 or more with resulting precipitant of the tetracycline as a base (link to pdf here).

A further study has been made on tetracycline and other antibiotics, assessing their stability in heat, UV and moisture:

Telia, A.C. et al. (2008) Research Investigation of the Effect of Moisture, Heat and Sunlight on Some Antibiotics Using Infra-Red Spectroscopy. Journal of Pharmacology 2 (3): 28-30. [pdf here]