Riddle mostly over hospital for doctors and nurses perhaps? (4)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

SIF{t} (riddle – sieve) [mostly] reversed [over], then H (hospital). I was baffled by the definition but have come up with ‘nurse shark’ and ‘doctor fish’. The first is familiar but the latter is not, however Wiki advises it’s actually called the red garra and it’s used in spa treatments where it feeds off the dead skin of humans. Yuk! If there’s another explanation, please let me know.