can you direct flow over the Bga? I knoe the filter for the 6.6 is orientated wierdly. lol
try dosing 1ml of H2O2 directly into the filter. (do this first thing when the lights come on)
increase to 2 mls the next day
3mls the third day...
until u get to 6 mls.
let me know how this goes >.<
if you see your cories react badly to this(this usually doesnt affect fish at all)
stop the treatment :)