The fish pose, or Matsyasana is the natural successor of the shoulder stand and it is recommended that you practice it as a counter pose to the stand. The pose implies a compression of the spine and neck as opposed to the stretch obtained while in the shoulder stand, bridge pose or plow pose.
There are several benefits of the fish pose. First of all it helps you expand the chest cavity, enabling the lungs to breathe more air and to become more accustomed to deep breathing techniques. The neck muscles and nerves also become stronger and more responsive while the spine increases its flexibility.
Begin the pose by lying down on your back with the legs straight and close together and with the spine kept straight, parallel to the floor. The position of the arms is also important: they should be straight, positioned under your thighs. The palms rest together, stuck to the floor while the elbows are as close to one another as possible.
Now to assume the pose.
Press your elbows downward toward the floor, touching the floor if you can, and arch your back.
Take a deep breath during this movement.
Take the weight of your body on your elbows, if possible (it may take time to get to this point), move your head backwards until it reaches the floor...and hold.
Exhale while holding the fish pose.
Relax your legs and allow your chest to expand while taking in a few long breaths.
To come out of the pose, try to slowly lift your head and only then release pressure from your elbows, allowing yourself to assume corpse pose.