Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Danios are social fish. Since you have only one he is probably hanging out with the other fish, which generally are lower tank fish.
I'll bet if you get some more (about half a dozen) they will stay in the upper areas.
Other reasons for fish to stay lower:
If there are no plants that are tall enough to provide some protection from predators (birds, cats) the fish will tend to stay lower. They do not know that there are no predators in an aquarium.
If you do not already have something tall, add some plants that either grow tall, or float.
If you are feeding a sinking food the fish know were to wait for food. They are being rewarded for staying low. Buy some food that floats for a long time. There are fine pellets that float like this.