Hello
I'm afraid you will struggle to get Fish validated as it is difficult to get species specific testing completed in this country. We use Neogen and they are unable to test for fish or oyster.
To get over this, we did similar to GMO and validated the clean for our worst case allegen, which is Sesame, using a recipe containing the highest levels.
In addition to swabbing points which had been risk assessed as the most difficult areas to clean, we also took samples of the sesame product and the follow-on product, whihc did not contain sesame, to a) verify that the test would identify the sesame and b) that the sesame did not carry over into the next product.
One word of caurtion, the testing of the surface swabs is very sensitive and Neogen were able to identify presence of Sesame down to very low levels. We had one reading of 40ppb on a swab, even though the sesame levels in the follow on product were below detectable limits. This makes interpretation very difficult, but all you can do in this case is to inform your customer of your findings and write this into your validation report and allergen policy.