My Salvinia natans keeps turning brown in the viv? its growth slows and starts to darken from a light green to a light brown. I have a double PC light, so plenty of light . I thought it was too much light so Ive only been using one, and they havent greened up? I have a few orchids (one that is about to bloom!), 3 kinds of neps. as well as several other plants, all thriving?