There are a few things you can add to the water in your Christmas tree to help keep it fresh and healthy. One is a cut tree preservative, which helps to prevent dehydration. Another option is a mixture of sugar, aspirin, and vinegar. The sugar provides energy for the tree, while the aspirin helps to reduce sap flow and the vinegar helps to keep bacteria at bay. You can also add a bit of bleach to the water to help kill any bacteria that may be present. Whatever you decide to add, be sure to change the water frequently and keep the tree away from heat sources.