2 or more nurses
Ethics has no plural. To refer to 2 or more ethical systems, you do what I've just done; did you notice?
2 or more salmon; they inherit the same (null) plural that 'fish' has in most contexts. (It's interesting that some fish don't: '2 mackerel' but '2 sardines', '2 cod' but '2 pilchards'; I don't know why this is, but hope someone will.)
Student or Learner