Saline solution like the kind for contacts, soak for about 5-10 minutes and then rinse it off with a little bit of water and a rag.
You can also try peroxide, but it may discolor dark carpet.
As always, test first.
To Mum with the fish. Try running a load of rags or towels through it with 2/3 a cup of borax and about a cup of vinegar in the rinse water. Also if your washer has a lint trap, check it out to see whether there is a lot of lint in there which is staying damp and trap odor.
