Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If the weather is OK, soak in bleach, then rinse in double dechlor, then spread it out and air dry. Any remaining chlorine will become a gas and leave.
You could follow up this with mineralizing (or at least pre-loading) the substrate so it is very fertile when you reuse it. This can minimize the water column dosing that may have played a part in the green water problem.