Subject: Why does our tetracycline stock precipitate?

We typically make up a stock solution of 1mg/mL tet in H20(d) and store at 4C. 
After a few weeks, we see yellow a precipitate at the bottom of the 50mL conical
tube and don't really know why.  We get about three weeks out of a stock
solution, but it would be nice to get a little more shelf-life out of the thing.
 Do you have any idea what we could do to prevent it, or (and frankly more of
interest) why this happens?

Thanks,

Josh

Re: Why does our tetracycline stock precipitate?