LADIES AND FISH ~ in MS Beinecke 408, fol.70r
.. in MS Harley 334..
item: Armoured fish, crested.
item: Paired fish – crested with uncrested
All these examples (except MS Beinecke 408) from the British Library’s manuscript collection.
If you’d like to find more examples of fish in medieval European art – search the “Medieval Bestiary” site by topic (here) and it will direct you to various European and American libraries. An ordinary online search for ‘fish in medieval art’ will doubtless turn up more.
and then, you might inspect manuscripts from Asia Minor and Islamic regions, fish represented in media other than manuscripts, and finally – if you’re really keen – fish before the middle ages.