At least five zebra danios should be kept together. Zebra danios are schooling fish, and they seek comfort and safety in groups. If you keep fewer than five zebra danios in a tank, they might become stressed, lose their appetite, become ill, or even show aggression towards other tank inhabitants.
Make sure that the tank doesn’t get overcrowded. As a general rule, each zebra danio requires two gallons of space in the tank. A small school of five danios needs a 10-gallon tank or larger to thrive.
If you’re keeping them in a community tank, don’t forget to consider the other tank inhabitants as well.