I have had my 31g freshwater tank for 2 months now. I put 6 cardinals and live plants in about a month ago. My fish stay close together, however, they hide all the time behind the plants and rocks. They don't appear to be "happy" in their new environment. They are quite skittish and don't swim around alot. They also don't come to the surface when I feed them. They don't appear to be eating (they don't come to the food). Also, I have a rusty-looking substance covering my white large rocks that are in there. Is there something I can do to eliminate this rusty looking substance? How can I make my fish happier so that they will swim around and eat?