Alvarez, you might want to start your own thread in the sick fish forum, gets better exposure that way.
From what you say, I think water quality is the prime suspect. Fancy golds are not know to be killers. Sure it is possible for them to out-compete a tank mate. Eg. the bubble eyes & celestrial eyes are almost blind, so is rather slow in finding food. If you keep those with a comet or even a metallic fantail, they won't find enough food & can starve to death.
This however, takes more than 2-3 days. A rapid demise of new fish (assuming that they are healthy to start with) is probably a water quality issue. You need to check your tank parameters (pH,
NH3,
NH4,
NO3, etc). Depending on your
PWC amount & how much mess the fish makes, your tank parameters might have drifted way off normal. The old fish, slowly acclimatizing to this, will be fine, but newly added fish will not survive.