Pinguipedidae (Sandperches)

Sandperches (Pinguipedidae) survive mostly on smaller invertebrates, but when fully grown they also hunt small fish and larger crustaceans.
In the wild they will often go after fish that dig in the sand, one can mimic this by placing bits of food in the sand and moving them around.

As they pose a threat to many of the shrimps and invertebrates which often inhabit aquaria, they are therefore best suited to a tank with more aggressive or larger fish.
If one organizes the aquarium to suit these fish, they can however be kept in relatively small tanks, as long as there is a large open sandy area. 

When a pair is wanted, it is best to buy them young or get two females.