Tom thats pretty dramatic a decline. .. has it ever been dewormed?
Also its been eating along fine?
Wasting away like that is normally a sign of three things.
1.parasites..almost always accompanied by feces thats not normal.
2. Cancer/ tumor
3. Organ shutdown from hormones. Given its a spotted fish..its likely.
Option 2 and 3 can occur quite dramatically... option 1.. can take months and months and almost always has white or yellow feces.
Al