I definitely agree that it is a ceratopteris of some sort. Although, I bet that plant is baby ceratopteris pteroides. If it is it will eventually grow fairly large with leaves raised above the surface.
I have some Ceratopteris pteroides. During the winter when it gets less light mine stays around 4" across. When it starts getting more light it revs into high gear growing over 12" across and the leaves change from solid to highly branched. Luckily it will have a bunch of baby plants I can detach from the edges of the leaves so when it gets too big I remove the monster mother plant and keep the babies.