I've grown hornwort, but it looks different to me.Hornwort (Ceratophyllum demersum)
It's not a fern, I don't see the Cabomba connection, however I can see it being some type of Ceratophyllum. All good thoughts!. . . it may be some type of Ceratophyllum, some type of other wort plant, slight possibility of it being a Cabomba sp? Maybe it's a type of fern very rare to the hobby?
Mainly found in brackish water if I am not mistaken. Only get 1-2 inches max.