One of my danios has lost weight and looks frail. He doesn't really swim much anymore and stays in one place. He's not really gasping for air at the surface, but he just looks like he's struggling. What should I do? Should I dispose of him or prolong his suffering? If I dispose of him should I just toss him in the trash? Sounds cruel, but I need advice. I don't want my fish to get sick or my tank to be contaminated.