Bettas

• Bettas are the most commonly kept fish for small tanks. They are also known as Siamese fighting fish, and for good reason. A betta won't tolerate another betta in its tank, and they will fight to the death. When kept alone, they are hardy and don't move very much. They can gulp air off the surface of the water and can live in very small tanks. While they can live in cups temporarily, a tank of at least a gallon is best for them. Smaller amounts of water can foul quickly and need constant cleaning.
  
  

Gobies

• If you don't mind the extra expense and monitoring of a saltwater tank, gobies can be ideal pico tank specimens. You can only have one goby per tank, but they can live in a partnership with certain species of pistol shrimp. The shrimp digs the burrow in the sand, while the goby keeps an eye out for danger. High fin red banded gobies, gold line gobies and wheeler's shrimp gobies are good choices for a pico tank. All of them stay under 3 inches.
  
  

Cardinalfish

• Cardinalfish are small, tropical saltwater fish from Asia. The smaller species such as orange lined cardinalfish, fowleria cardinalfish and long spined cardinalfish are good pico tank inhabitants. None of them grow longer than 3.5 inches. They are slow moving and peaceful, but with a pico tank, you don't want more than one per tank.
  
  

Ghost Shrimp

• Ghost shrimp aren't fish, but they can be kept in small tanks with other fish. These tiny freshwater scavengers stay on the tank bottom, keeping it clean by eating leftover food. They are called ghost shrimp because their bodies are almost transparent. You can see their food in their stomach after they eat. Ghost shrimp grow to about 1.5 inches. A betta and a ghost shrimp make for a good pair. The shrimp will eat any food that sinks to the bottom.