In article , NetMax
says...
What I would do is brine bath. Prepare a saline solution and have a
large soft sponge on hand.

Well, I've done the business, and the fins have cleaned up really well.
There's still a little stubborn staining on the body near the tail, and
no doubt that'll go after a couple more treatments. The fish hardly
struggled at all during the treatment. Thank you all for your
suggestions.

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Regards,
John Patmore -