What should I say?

Got complained again by customer for serving them salmon with silver skin. The same old "I've been eating sushi a lot/I'm from Hawaii, this is unacceptable" How should I respond without being condescending but also make my point?

Got complained again by customer for serving them salmon with silver skin. The same old "I've been eating sushi a lot/I'm from Hawaii, this is unacceptable" How should I respond without being condescending but also make my point?