PVC Tank Adapter,
2-1/4" Female Fine Thread used by General Ionics ( Purionics ) valves
x 2.5"-8UN Male ( type of threads used with standard industry tank opening ).
This will allow you to move / reuse your old / existing General Ionics ( Purionics ) valve head ( that uses 2 1/4" fine thread valve )
to a "Standard" replacment tank ( that uses 2.5" x 8 NPT opening ).
You can Re-Use your resins in most cases, SEE: Resin Replacement Guide
And the brine tank can be re-used,
For Valve Replacement Info See:
How to Replace Your Old Valve