Hi,
Joomfish 2.5.1 for joomla 2.5 is not yet stable.
I would recommend you to use Falang instead until Joomfish is stable on Joomla 2.5. The latest version of HikaShop is not yet compatible with Falang but it will be in next version of HikaShop. If you can't wait until then (less than 1 month), you can use our contact form
www.hikashop.com/en/support/contact-us.html
to request a pre version of next release of HikaShop which will support Falang properly.
We actually don't have access to that version of Joomfish and never tested it with HikaShop. However, that error happens in Joomfish, so the best would be to report it to Joomfish developers.