Hi, Two weeks ago, I purched a bunch of giant hygro. It was doing well. However, this week, the leaves are turning yellow, and the stems are turning brown. Some have meilted away. This also happened to the giant hygro I bought last year. After three weeks i had it, it also melted away. Do I missing some kind of micronutrient in the water, or it is not a true aquatic plant? Does any one know?