I only have succulents and they don't make rizome as they propagate themselves from the mother plant's roots. I swear it just randomly appeared in my tank as I never saw it in the 5 weeks that I watched my tank cycling and when I first added my 2 clown fish. However, I do have a planted tank, but none of the plants that I have in have a rizome that looks like that.Looks like some terrestrial plant, are you sure that this frag don't droped iinside the aquarium? have you other plants in your house?