Changing Sous Vide water
The water frequently sits in the danger zone for bacterial growth, and even though it is a sealed bag, fats can migrate through the plastic (not considering contamination to the outside of the bag either from cross contamination or from juices drawn out during vacuuming/sealing).

I would suggest changing the water if:


is looks dirty/cloudy
it smells like anything other than water


Additionally from the PloyScience site:


  Occasionally or when a cooking pouch may have leaked, simply replace the water with a vinegar/water solution and run unit at 160°F for 25 minutes to remove any residuals or lime.


So you can probably throw a little vinegar into the water to up the pH and make it more hostile to bacteria (though it will smell up the house when you use it).