In the refrigerator, store fish on top of ice rather than directly on a shelf. Ice keeps fish near 0°C / 32°F; the refrigerator’s own temperature of 4°C / 40°F is not cold enough for this product. The setup is simple: a perforated container or colander, with another container underneath to catch the meltwater. The meltwater must not touch the fish, or its texture will become soggy and its flavor will leach away. The fish’s temperature matters, not the refrigerator’s.
Fish keeps best in conditions close to the temperature of the sea, which is lower than a refrigerator can provide. The melting point of ice, 0°C / 32°F, is the coldest temperature at which fish can remain unfrozen; at this temperature, both enzyme activity and bacterial growth slow considerably. A refrigerator’s regular shelf is around 4°C / 40°F, which is not cold enough for fish. That 4°C difference (about 7.2°F) can make a 1-day difference in the fish’s freshness. This is exactly why fish markets display fish buried in crushed ice, and the setup at home is a small-scale version of that.
The setup consists of two containers. Place a perforated container, colander, or strainer on top, and put the fish inside it. Set another container underneath to catch the meltwater. Cover the fish with ice and cover the whole setup with a closed lid or plastic wrap. The key point is that the meltwater must not touch the fish: fish sitting in its own meltwater becomes soggy, loses flavor, and spoils faster. Empty the water underneath once a day and replenish the ice.
Where you place the container also matters. Put the fish on the bottom shelf of the refrigerator; it is the coldest area, and any possible leak will not drip onto food below. Do not store other food above it. Keep the container covered because fish odor spreads quickly in a closed refrigerator and can transfer to odor-absorbing foods such as dairy products and eggs. If storing a whole fish, remove its internal organs and gills first, since those are the parts that spoil first.
Even with this setup, storage time is short. Cleaned fresh fish can be kept on ice for no more than 1–2 days; the time may be even shorter depending on the species and when the fish arrived at the market. Fatty fish spoil sooner than white fish. Fillets and ground fish have an even shorter storage time because they have more surface area. If you need to store it longer, freezing is the right choice; make that decision on the day you buy it.
You may not always have ice on hand. In that case, store the fish in a covered container on the bottom shelf; folded paper towels under the container will absorb any liquid and help keep the fish dry. Ice cubes from the freezer can also be put in a bag and placed on top of the fish. Whatever setup you use, do not leave the fish at room temperature; fish left on the counter does not regain the time it lost, even if you put it back in the refrigerator. To keep the odor from spreading through the refrigerator, you can place a slice of lemon or a few sprigs of parsley in the container.