Since I dont want to waste the database with html garbage...- So you want to waste server resources, each time converting what is posted on the fly when user loads the page? I would recommend to think deeper until you understand why we use (and why it's useful) another approach.
I suppose
geshi is "
Generic Syntax Highlighter" so it's more than obvious that you need to put such functions in encodeBB() and decodeBB() of bb_codes.php. However I can't tell you exactly what could be wrong on your side implementing it... custom solutions usually are implemented/investigated by us only for a
fee.