Well, when Cole rolled in the residue from the septic truck hose, I took a fresh bottle of hydrogen peroxide and used that to clean him up. It took out the black and really helped with the smell. Not sure if it will work with fish odor, though, as I believe the "fishy smell" molecule is smaller and less reactive than septic smell
If you do
try peroxide, remember to rinse him off thoroughly with water afterward as the chemical can be pretty harsh...
Another thought--how about a lemon juice rinse? Lemon juice takes that fishy odor off hands
And it makes hair really shiny!