pigeon blood discus

Hi there, Can anyone help me. My discus fish is sick. I have a 20 US gallon tank. In which I have 2-3 inch sand layer at bottom. Having few plants 1. Ludwigia Ovalis - 5 stems 2. Red Cabomba - 10 stems 3. Rosefolia - 3 stems 4. Bacaopa Cariliona - 15 stems 5. Java Fern - 1 plant 6. Amazon Sword...