Notes on a fish? (5)
I believe the answer is:
scale
This might be a double definition.
'notes' is the first definition.
(musical notes ordered by pitch)
'a fish?' is the second definition.
The answer and definition can be both animals as well as being singular nouns.
I also know that 'fish' relates to this answer.
'on' is the link.
(Other definitions for scale that I've seen before include "Notes < magnitude" , "range" , "Climb - balance" , "plaque" , "Relative size (of something)" .)