Scale a fish by scraping from tail to head, against the direction the scales lie. Use the back of a knife, the edge of a spoon, or a fish scaler; you do not need a sharp blade because the goal is to lift the scales, not cut them. Since scales fly in every direction, work in a basin of water, inside a bag, or in the sink. Not every fish needs scaling: skip this step for species with scaleless skin.
Scales overlap like roof tiles, lying from the head toward the tail. For this reason, scrape from the tail toward the head; scraping the other way will not lift the scales and will only press them down further. Hold the fish firmly by the tail. If needed, rub salt on your fingers to improve your grip. Use the back of a knife, not its sharp edge; the edge can cut the skin and tear the surface of the flesh. The edge of a soup spoon works just as well.
As soon as they are scraped off, scales fly in every direction and spread throughout the kitchen; cleaning them up afterward is the most tedious part. There are three practical solutions. First, scale the fish in a wide basin filled with water; the scales fall into the water instead of flying around. Second, put the fish inside a large plastic bag and work with your hands through the opening. Third, scale it in the sink, but do not run the water, because splashing water can spread bacteria from the raw fish. Whichever method you choose, clear the counter first. Wearing an apron is also part of the job; scales stick to fabric and are hard to remove in the wash.
Not every fish needs scaling. Mackerel, chub mackerel, and salmon have very small scales that cannot be felt by hand, so skip this step for them. Eels and some other species have a slippery coating instead of scales; rub it off with coarse salt. Scales on small fish such as anchovies and sardines come off during cleaning anyway. By contrast, sea bream, sea bass, common pandora, and mullet must be scaled; their scales are hard and are not edible when cooked.
The order of steps matters too: always scale the fish before cleaning its belly. If you scale a fish after opening its belly, scales can get inside and be difficult to remove. When you finish scaling, hold the fish against the light; any remaining scales will shine. The places most often missed are the bases of the fins and the underside of the belly, as well as around the gill cover; check these spots separately with your fingertips. Then rinse the fish with cold water and pat it dry with paper towels.
If you are cooking the fish with its skin on, scaling is essential; scaled skin will neither turn crisp nor be pleasant to eat. If you plan to remove the skin, there is no need to scale it, which saves time because the scales come off with the skin. When asking a fishmonger to clean it, mention this too: ask for scaling if you will keep the skin, and say it is unnecessary if the fish will be filleted. Finally, do not leave scales and scraps exposed; put them in a sealed bag and throw them away, since fish scraps quickly develop an odor in the kitchen. If you buy a fish scaler, choose one with a covered collection chamber so the scales go straight into it.