wrote,
I will look into
the gill parasites and flukes more...

Is one Pectoral fin being held tight against it's body - could be the
start of a parasite called Costia. An early symptom is little red
hemorrhages (blood spots under scales) just below the pectoral fin (fin
on side of fish). The fish holds the fin tight against it's body while
the other pectoral fin on the other side of it's body still works - the
fish tilts to one side. Treated with QuickCure or any other
formaldehyde/malachite green anti-parasite medication...
How about constipation - see any stool (fish waste)? A teaspoon of
Epsom Salts per 5 gals. makes a good purgative and won't harm the other
fish...
Still thinking at this end - any changes yet? ................. Frank