As the man said. However, if you are willing to put up with some handwaving and vagueness, there are certain classes of compounds that are prominent in the flesh of aquatic creatures, so thay commonly smell like that when they go even a little off. They are largely straight-chain unsaturated alcohols, aldehydes and fatty acids with double bonds in various places. There also are some sulphur compounds like dimethyl sulphide, butyric esters, and ammonium compounds, most notoriously TMA (trimethylamine).
Some of those are from chemicals peculiar to the fish, but others come from the algae and similar creatures eaten low in the food chain and passed on up as larger creatures eat smaller ones and accumulate their tastes and distastefulnesses. The marine ones tend to differ somewhat from the freshwater ones, which tend to have a "muddy" taste, which to my taste passes on to flavour the flesh of water birds too. In fact, in the days that chicken feed contained a lot of unrefined fishmeal, chicken tasted quite nastily fishy.
Two compounds that give freshwater creatures their muddy taste come from microbes like cyanobacteria, notably Anabaena, which also can taint drinking water. Geosmin can be quite pleasant when it occurs by itself as the "rainy" smell of earth after a drought. It is a naphthalene compound produced by various microbes. Another, nastier, corkier smell is 2-methyl isoborneol. The compounds are pretty well ubiquitous in freah water and fairly stable and oily, so they are hard to get rid of and they accumulate.
Did you thnk that only synthetic compounds were persistent?
Cheers and I hope what i told you will help you enjoy your fish,
Jon
