I have two platy fish in my tank with others Gouramies, neon tetras and swordtail. From last two weeks i have noticed that one of the platy is always sitting at the bottom of the tank or behind the plants. It has also lost alot of weight, its like half of the size it was before. When i add food it comes at the surface of the water but does not eat much. Any idea what is going wrong??? Regards