Brine Shrimp (artemia)
Brine means 'salt solution'. Well shrimp is shrimp. Live brine shrimps comes in many sizes, the smallest sized ones are tiny because they are newly hatched. The big ones are adults and can often be seen carrying eggs (cysts). Other than live brine shrimp, you can also buy frozen ones and even brine shrimp eggs to hatch yourself (using salt water).
Brine shrimp is a favourite food for fish. In fact, there is hardly any fish that will not eat them.
If you get a large package of them, you can store the excess in the refrigerator (not freezer!). In the low temperature, they will hibernate (go to sleep) and sink to the bottom but they are not dead. When a portion is taken out and put at room temperature, they will wake up and get active again. Using this method, you can store brine shrimp for long periods without them dying out.
If not stored in the refrigerator, you should keep them heavily aerated.