I wouldn't use Christmas trees. Use come PVC pipe or some Bamboo. THe trees will be MAJOR HANGUPS and they have so many limbs the fish really can't get in them that good. Go to your local Aquarium and see how they have ot set up and try and imitate that. it's usually limbs or stumps that are tight cover but open enough for the fish to get in and out of it, etc.
Just my 2 cents.