Hey guys, I'm going to open this up for debate, as things are getting a little off topic on my personal system discussion. There have been questions as to the safety of charcoal as a media, and I'm sure there are questions out there regarding it's environmental impact. So let's let fly with the questions and hash this out. I'm definitely not the expert here, and I am hoping George will weigh in on the issues.
Personally I decided to use charcoal based on the fact that I have free access to wood, and needed a cheap and light media for use in a GB supported by a light stand that's not meant to support a lot of weight. I know that activated charcoal is used in aquaria as a filter media, so I'm pretty sure it's safe. That said we all know that many things from tropical fishkeeping don't transfer to aquaponics very well.
@Bob...Like Jesse says, ash content will certainly affect pH values (obviously), and again, this is mostly related to the pyrolysis temps used. Acid washes are used to partially remove this ash content (inorganic Mg and K) which plug up all those small pores...