I don't know if a milk marinade would help remove the ammonia smell from fish, but milk does wonders if you soak venison steaks in it.

Try getting some catfish from the store (farm raised cat), peel off any skin fragments left on it, and soak it all day covered in beer (cheap beer is fine). As the oil rises, skim it off; replace beer if you have to. Bake it, treating it as you would any good white fish (cod, etc.). If you don't like the taste of catfish, this will pleasantly surprise you.

Now, I happen to LIKE catfish, rolled in cornmeal and fried in lard, with a good coleslaw and some COLD beer...mmmmmmmmmmmmmmmmmmmmmmmmmmmm.