I have smoked and grilled fish in drums, Eggs and Little & Big Chiefs without any residual odors or flavors from the next cook. I will allow the fire to burn after I'm through with a load of fish, and I toss on a couple more splits (or a partial pan in my Chiefs) just in case.
All that said, I have heard a lot of folks claim WSM's will pick-up a fishy flavor.
KCBS - CBJ
Big Green Eggs, Big Drum Smokers, Big Chiefs, Weber Smokey Joe "Custom Tall Boy"
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Barbecue is not rocket surgery