floating water sprite wisteria loosing roots

I have a big bunch of floating water sprite/wisteria, the plants used to have long bright roots,when the tank was new, now tank is almost a year old and all of them have only short brown roots, the plants still grow and sprouting new plants, but not like before, what could cause the lost of roots? can it be a problem with nutrients in water or there is too much nutrients and they don't need those roots?