Deterioration in White Fusion Calathea’s health can be because of several reasons. First, look for the symptoms for the correct diagnosis of the problem. If your White Fusion leaves have excessive yellowing most probable cause is over-watering. Reduce the water input to plant and water only when the soil has partially dried. Please ensure the water drain hole in the pot is not blocked. Root rotting can be involved if the yellowing of leaves is more evident at the bottom near the roots. Inspect the roots and if you notice a sign of rotting, remove the damaged roots and repot the plant in fresh soil. If the leaves’ edges are brown colored, the plant is not getting enough humidity. Increase your indoor humidity level to resolve the issue, or place the plant in a more humid area like a washroom. If the leaves midspan is discolored, too much sunlight could be the reason.