Your filesystem has a size of 5.2 TB, only 343 GB used (6%)  There is no way to shrink a FS, except host based copy of the data inot another FS and destroying the original one, sorry.
Hi, hope someone can help whilst away on holiday one of my colleagues attempted to increase our user homes dirs quotas which he thought he did but all it seems to have done is filled the storage pool. (Not using tree quotas) I reset the quota to it's orginal settings but it has not restored the storage pool space.  The user homes dir is was using around 400GB (around 200 users with a 5GB hard limit) of the storage pool and would expand when needed by 20GB when the quota was increased it filled the storage pool of 6TB yet is still only using 400GB would appreciate any help recovering 5.6TB of storage pool. I am sure i am missing something.   
Quota setting changed & filled storage pool