Ive heard just soak it in %5-6 Hydro peroxide, some people dilute about 2tsp of it in 500ml water. Either that or soak it in KMnO4-mix 1-2 grains-ONLY GRAINS not a whole teaspoon- with 1l water
As for the medium, guys at the local OS always use a medium grade (here it would be grade 2) orchid pine bark, or they mix 1 part chopped sphagnum with 2 parts bark, and then they maybe add half a cup of styrofoam packing pellets.