Nixies are found in fresh water pools are streams. They can be identified by the fact that they are always dripping water from their clothes and hair and have a greenish color to their skin.  They can be on land and in the water and have legs rather than tails like merfolk.  Nixies are like treefolk whereas they can't venture too far from their body of water. They are very curious about land people. Sometimes a Nixie will coax a human into their water, but will not harm them. They sometimes like the company. Nixies are herbivores and are friends with the animals they dwell with.
* photo courtesy of Arthur Spiderwick's Field Guide