What is Bio-char made of?
It has been written about here before but not a lot. The only problem I recall with it is that when used as surface media, it wicks moisture to the surface. Otherwise, it should make a great nitrifying media due to its porous structure with lots of surface area.
Robert, generally I make mine from various types of wood picked up off the curb in the city but it can be made from any biomass material. It's simply charcoal made and used for agricultural purposes. Definitions vary, depending upon source. I've gotten good results using it in my soil garden but haven't used it in aquaponics yet.
Biochar - I wouldn't count on it as a long term buffer for PH. Nitrification will always drive PH down and the liming effect from adding biochar is temporary, in my experience.
I hear there is a current discussion on the topic on linked in between Tim and Murray