Stringy white poop... parasite?

[stricknine]

I had 2 banggai cardinals purchased from the LFS, one died a week later and the other isnt looking too good. This was my original post;
About a week ago I added a pair of benggai cardinalfish to my tank. Right off the start one of the two cardinals did nothing but hide, didn't eat much, turned brown in colour and died sometime today. The 2nd cardinal, close to adult size, seems healthy but it's body and dorsal fin is taking on a goldie-brown colour. Again water parameters are great, feeding okay. The remaining fish appears to be active with normal behaviour.
The adult remaining is now showing the same symptoms; brownish, loss of appetite, lazy, and white stringy poop. One of the responses to that post mentioned the poop and a parasite but no details.
Also, water parameters are good.
Does anyone know what could be wrong and what I can do to fix? Also, can whatever it is be contageous to the other fish and corals?

THANKS!!!

 

[paulamrein]

Could be cyanide poisoning. IF you are not familiar they use cyanide to catch some fish in some parts of the world illegally mind you. There are regulations but they still fall through the cracks. It's quick and kills within a 10 days to week and a half range. Unfortunately, if it is in fact cyanide poisoning, it's a goner. The thing is, is that bangai are now tank bred, so it may be a worm or something as well if the bangai was tank bred.

 

[paulamrein]

To answer your follow up question, it's not communal.