How to describe the smell of fish
I would use the adjective "fishy," actually. The smell of fish is pretty universally known, and that's what we say. One can even say that other things smell fishy.
You could use one of the following words: foul, noxious, putrid, rank, or reeking.
The smell of fish comes from a similar process as the smell of urine - the breaking down of amines.
So you could call the smell
- aminic: relating to amines
- ammoniacal: relating to ammonia
I find acrid a useful adjective, though in my mind, that can also mean smoky.