Kimberly,

We use a modified Zenker's fixative called Helly's:
     20,000 ml   Deionized Water
       1,000 gm   ZnCl2  (zinc chloride, CAS# 7646-85-7)
          500 gm   K2Cr2O7 (potassium dichromate, CAS# 7778-50-9)

Add 1 ml of 37% unbuffered formaldehyde (CAS# 000-050-000) per 
20 ml of Helly's solution prior to use.  This formulation replaces the
mercuric oxide component in the Zenker's with less toxic zinc chloride.
Still a 24 hour shelf life once formaldehyde is added.  

Dodi Borsay Horowitz
U.S. EPA
Atlantic Ecology Division
Narragansett, Rhode Island
borsay.dodi@epamail.epa.gov