Plain tap water is all your Christmas tree needs.
You do not need to add sugar, aspirin, bleach, soda, fertilizer, or other homemade mixtures to help your tree stay fresh.
The most important thing is to make sure the base of the trunk stays submerged in water at all times and that the stand never runs dry.
Should I use warm or cold water?
Water temperature is much less important than keeping plenty of water available.
In our experience, we like to use warm, but not boiling, water when first setting up the tree, but plain room-temperature or cool tap water is also fine.
Do Christmas tree preservatives help?
Commercial Christmas tree preservatives are generally not necessary for a fresh-cut tree.
Proper care matters much more:
Get the tree into water as soon as possible after a fresh cut.
Check the water level every day.
Never let the stand run dry.
Keep the tree away from fireplaces, radiators, heating vents, space heaters, and prolonged direct sunlight.
If you're deciding between buying a tree additive and simply keeping your stand full, consistent access to plain water is the priority.