Dying Valisneria

Can someone please offer ideas or suggestions? I am relatively new at
raising aquatic plants. I have recently set up a new tank,(120gal). I
transferred all fish and plants from my old 35 gal. to the new tank. All
plants seem to be doing well except Val. spiralis. It starts Turing
brown at the tips and works its way down the leaves, also the roost
appear to turn brown and rot. I have replanted twice with new plants
with the same result. Any help would be greatly appreciated.
                                             Mace Arnold