The best thing to add to water is an additive such as TreeKeep or Oasis, which is specifically designed to provide nutrients and moisture to Christmas trees. You can also add some sugar to the water to help keep the tree hydrated and healthy. Sugar is a natural food source for Christmas trees and will provide them with the extra energy they need to stay fresh and vibrant during the holiday season. Additionally, you can add a small amount of bleach to the water to prevent bacteria from growing and to keep the tree from getting too dry.