So all fish parts are restricted to water? Mermaids can exist outside of waters (though not efficiently moving, hence low stats?)
Look at the air image, it's distinguished from fishtails. Life already has frogs, which are water creature after all.
The image of earth would be without fishtails.
A creature whose major thematic trait is a fish part would be Water.
Depends on the mythos. Most mythos that include mermaids have them kept in the water either through gills or having to remain moist.
The Air image was the best of the non water images. However I would expect bird parts not fish-like parts on a part animal humanoid from Air.
Frogs are amphibians and are often on land instead of in the water. They are more thematically linked to the jungle than to Water or Earth (hence Life)
If you go multielement I suggest:
(Bird) Aven/Raptorian/... [too many names to list]
(Horse) Centaur or (Bull) Minotaur or (Goat) Satyr or (4 legged animal)
(Snake) Naga
(Fish) Mermaid