The substance that should be added to the water in order for the bubbles to reappear is carbon dioxide.
During photosynthesis, aquatic plants use carbon dioxide, sunlight, and water to produce glucose and oxygen. The bubbles observed on the plant's leaves are likely oxygen being released as a byproduct of the photosynthesis process. Without an adequate supply of carbon dioxide, the plant won't be able to carry out photosynthesis effectively, and thus the formation of bubbles will decrease or stop.