Answer: Sarvangasana (to decongest the lungs) and shavasana after that, to make your mind calm.
Query: Why is the fish a counter pose to the shoulderstand?
This should be a question on which u all must expend some thought... Most lazy instructors have no sequence in place, including in your yucky power yoga sessions (that any case is not yoga:). A sequenced yoga class is what makes you feel good. Apart from that feel-good it means the flow of waste from your body is being thought about. It is like doing massage -- the direction of waste and energy flow has to be manipulated in a good sequence... which explains why I worship the Sivananda school sequence -- there is nothing more compact than that... and perfect... all schools, including world class ones (barring maybe the Mysore school and Ashtanga yoga and Bihar school in theory) bother much about sequence...
so for the original question: why is the fish a counter pose to shoulderstand?