Hi,  The method we use for zinc formalin is as folows:

18 L water
 2 L formalin
200 g zinc sulfate

we let the water and formalin "age" for about 2 days, then add the zinc and 
stir, stir, stir...  Or you could remove about 1L of formalin mixture, add 
the 200g zinc , let sit for a day or two until dissolved, then pour the 
solution back into the large 20L container..

Jeanne Godine