I have been doing this for quite a while. I just make sure I rinse everything very well before I use it. Some deli shops will spray this stuff with a preservative.
+1 on rinsing well. I didn't rinse very well on my first batch and although I didn't have issues with stuff in my water I did have issues with cloudy water after feeding but it would clear up almost immediately. Just outta curiousity...what would happen if you got preservatives in your tank?