Pajama CArdinal not looking 'right'

[euromomtx]

I have 4 PJ Cardinalfish currently in my quarantine tank that I received on Wednesday.
(One arrived dead)
The other 4 seemed to be doing fine, not liking pellets but eating cyclop-eez and frozen food.
Today one of them seems unhappy, top fin down and his tail is all read (they're usually see-through and dotted)
Ammonia and Nitrite are 0 but Nitrate is detectable.

 

[euromomtx]

it died...

 

[francis1123]

i would guess some type of fungal fin rot. possibly it was being bullied by the others. it is pretty common for cardinals to pick out the weak one in a group and kill it.