I’ve had them for at least 6 months- I bought them at a reef convention ant they were fully extended even after the stress of a convention. When I got them home they maybe extended a millimeter.
Assume by 'salt' you mean a specific gravity of 1.025? Your other parameters would be helpful to know as well, calcium, PO4, etc. Are you feeding at all? Some people recommend Fe and Manganese supplements for goniopora. Hope this helps
It would also be helpful to understand how much flow you have over the Goni. Try moving the Goni to a spot where there is less flow; or adjust your flow lower over the Goni if possible.