What The Experts Say!
Most experts insist that good old water is all you need to keep your Christmas tree fresh through Christmas. Other scientists say there is an additive that will increase both fire resistance and needle retention. You decide.
One thing to remember what will effect water uptake. If your tree is more than a day old you might want to saw an inch "cookie" off the tree's trunk bottom. Even a small sliver shaved off the tree butt will help.
This process freshens the trunk and allows water to be quickly taken up to the needles for continued freshness.
From NCTA's FAQ:
(Q) Should I add anything to the water in the Christmas tree stand to help it stay fresh?
(A) NCTA does not endorse any additive. They insist your Christmas tree will stay fresh with just plain water.