I'm almost certain it was an internal bacteria, the thing is UK meds seems to do nothing to cure it. I'm not sure where you're from but if not from the UK perhaps you have better meds to treat it. Mysterious deaths are generally considered to be internal bacteria.
A lot of my fish also went a darker colour which makes me more certain that it was an internal bacteria.
Good luck!
Oh and the cause I believe was an infected krib who I had recently bought, I quarentined him for 2 weeks but this was obviously not enough and as soon as i moved him into my main tank his symtoms started and then 1 by 1 11 more fish died!