A Nixie is a female Nokk, a species of Germanic merfolk (The men are called Nix).
They are river creatures, as opposed to living in the sea, and are closely connected to both the Rhine and the Danube.
Nixies can ‘walk’ on land, but are identified by the wet hems of their long dresses. The fish tails don’t really dry off easily, I guess.
In addition to marketplaces, Nixies also show up in tragic German operas, like Wager’s Ring Cycle. In those operas, the Rhine maidens who guard the Rhine gold that starts out all the ‘ring’ trouble are comparable to Nixies.
A Field Guide to Demons, Fairies, Fallen Angels and Other Subversive Spirits
by Carol K. Mack, Dinah Mack