Youll want to use bleach
Prepare a ten percent bleach solution by mixing
nine parts water with one part bleach in a clean
bucket or container (example: mix 9 cups water
with 1 cup bleach). Submerge the aquarium
items in the solution and soak for ten to fifteen
minutes. If cleaning an aquarium, simply fill the
aquarium with the ten percent bleach solution.
Following the soak, drain off the bleach solution,
rinse the container, and refill it with clear water.
Place the items in the clear water and allow to
soak for another fifteen minutes in the water.
Rinse well with clear water and allow to air dry.
Copyed from
http://freshaquarium.about.com/od/maintenance/a/bleach.htm
