I use a 1/2 teasppon of bleach to 1 quart of water, I let it dry in the sun then give it a good rinse and redry in the sun. My bator is stryofoam to and 20 some years old. I have gotten good hatches out of it everytime, except one, but that was for poo covered eggs that exploded for days. Cleaned the same way and had 60 eggs set and almost all 60 eggs hatched.