Soap won't hurt your cast iron

Yes, use soap. The whole “never wash cast iron with soap” thing is a myth from the lye-based soap era. Modern dish soap is mild and won’t strip your seasoning.

For fish or anything strong smelling, just wash it like a regular pan—hot water, soap, a stiff brush. The smell comes from oils that stayed on the pan, not some deep flavor absorption. Soap lifts those oils. Rinse well, then put it on a burner over medium heat until bone dry. That heat alone will burn off any lingering odor.

If you want extra insurance (or you’re really sensitive), scrub with coarse salt and a little oil before washing. The salt abrades without damaging seasoning and pulls out stubborn smells.

One thing not to do: let it sit in the sink. That’s how you get rust and a permanent fish funk.

Just wash, dry on heat, and maybe give it a quick oil wipe. Done.