One alternative is to cut the spot out and replace it with the same carpet from a closet or under something.......you will certainly get rid of the smell that way. You may need to remove the pad as well.

I would certainly try what the other folks suggested, first, then as last restort, get out the carpet knife!

Good luck.

P. S. I never serve fishy stuff to my cats cause I hate the smell both before and after....ahemmmmmm. So they get chicken, turkey, beef etc. but never any fish.