I don't think it's a red phantom tetra.
The fish either has air trapped in its digestive tract, or has a swim bladder problem.
Stop feeding dry food for a few days and use frozen (but defrosted) instead. If the fish still floats after being fed frozen foods for a week, it has a swim bladder problem. If the fish swims normally after the frozen food, it is air and you will need to reduce the dry food in its diet, or wet the dry food before feeding.