This is a Thor amboinensis which is more commonly known as a "sexy shrimp"  It gets the name from the way that is always moving and flipping its tail around, even when its legs are not moving.  It is a very entertaining little show to watch and would be perfect for a nano tank.  This shrimp is around 3/4 inch  to an inch long.  It is very rare for us to see these shrimp around since they are usually much smaller and obviously very hard to see in the wild.  If you do a search on the internet, you may even find a video of this type of shrimp doing its little dance.