Dartfish (Microdesmidae) are recognized by their unique torpedo like shape. All Dartfish need hiding places where they can withdraw to when feeling threatened. They do not normally naturally dig their own holes, but use ones made by other creatures.
They are generally a popular aquarium fish, relatively easy to keep, so for a small tank with peaceful fish the Dartfish is the obvious choice.
They can easily feel threatened by larger more aggressive fish with the result that they cannot feed enough and die by lack of food. One has therefore a greater chance of success when they are kept with smaller, peaceful, fish.
After first introducing a Dartfish, it is advizable to dim the light, so it can get used to the tank in peace.