Not looking good!

[Escher]

One of my young mandarine has been withdrawn for the last few days and lost some of its colour. Tonight I noticed it has a swollen abdomen and gills open wider than usual.
Water parameters are ok (I think)
Ammonia Nil
Nitrite nil
Nitrite 5ppm
pH 6.60
temp 30deg

although I had a (gradual) pH drop to 6.0 last night. (before 25% water change)
The tank has received a course (3 doses) of metro and salt as problems with juvenile diamond were not resolved (& still isn't: 2 dead, 1 on its way, the other perfect! - but different symptoms alltogether).
I have attached two pics (not the best unfortunately as the fish would not pose ); the one with two fis shows a sibling behind it with normal colouring for comparison.
Any idea what could be and course of action.
Thanks to all.
Gianluca