[wieb]

I am no expert on potting media for orchids. I have maintained aquariums for a couple of decades. And I know a bit about chemistry through college and my job.

Removal of chlorine from water is essential in the aquarium hobby. Aquariums stores have "water conditioners" that contain sodium thiosulfate which rapidly neutralizes chlorine or residual bleach. It is harmless to fish in small quantities (you add it during water exchanges).

If you are going to sterilize with bleach, be careful and neutralize it well. Bleach is used to sterilize aquariums and it is really hard to get the odor out (it seeps in every imaginable pore available), even after rinsing many times. The odor means the bleach is still in the system. I imagine the lava rock must have millions/billions of pores for this bleach to seep into and hide. I would guess that you could rinse it for 20 minutes straight and you could still smell the bleach. I would rinse it well (several minutes of continuous running) and then I would soak it in a dilute mixture of this water conditioner for about 30 minutes with occasional stirring. The water conditioner is typically used at about 1 ml/gallon when neutralizing tap water -- I would bump it up to 5 ml/gallon for neutralizing residual bleach. Then pour off the neutralizing solution into the sink and flush again for a couple of minutes with fresh water. The bleach odor should be gone. If not, repeat the treatment with a higher concentration (maybe double). The sodium thiosulfate is free rinsing and any trace amounts would not hurt orchids, I am sure, since it doesn't harm fish.

By the way, bleach does not dissolve most hard water deposits and salts. White vinegar (acetic acid) is good for this. A 50%/50% mixture of white vinegar with water is good at solubilizing hard water deposits and is also used throughout the aquarium hobby. But then you would have to worry about attacking the lava rock; rinsing from the pores; resultant pH of the medium. Probably the best advice is what I see all over the forums – don’t use lava rock if you have really hard water because it will collect the hard water salts on it.

Here are two links, one talking about bleach neutralization and the other about removing hard water deposits, in case you are adventurous.

http://www.newton.dep.anl.gov/askasci/chem00/chem00046.htm

http://www.howtogetridofstuff.com/stain-removal/how-to-get-rid-of-hard-water-stains