Your goldfish died of white spot and unless you treat the shubunkin with a proprietary white spot remedy quickly it to will die.
White spot (or Ick) as it is called is a stress related problem, endemic to coldwater, tropical and marine fish.
The move from shop to tank can set it off, the fact your tank is still very new with less than optimum water quality probably exasperated the problem.
Once the tank is treated, (Ick treatment is usually copper based) the fish should recover. It would be a good idea to treat the whole tank prophylacticly with a good white spot treatment whenever you introduce new fish, to ensure they remain free of this most common of all fish deseases.
