To get a wonderfully crispy crust when shallow-frying fish, pat the fish really, really dry, give it a light coat of flour or a breading mix, and then fry it in hot oil over medium-high heat.
Moisture is the archenemy of crispiness! Before you even think about coating your fish, grab some paper towels and pat those fillets completely dry. This gets rid of any surface water that would turn into steam, which makes your fish steam instead of crisping up. Next, a light dredge in seasoned flour, cornstarch, or a breading mixture (like flaky panko breadcrumbs) creates a fantastic barrier between the fish and the hot oil, helping to form that lovely crispy outside. Make sure the oil in your pan is properly hot, around 175°C (350°F), before you add the fish. If the oil's too cool, your fish will soak up too much oil and get greasy. If it's too hot, the coating might burn before your fish is cooked through. Cook on one side until it's beautifully golden and crispy, then flip and repeat for the other side.
Don't cram too many fish pieces into the pan at once! Overcrowding drops the oil temperature, which can lead to steaming instead of frying, and a less-than-crispy crust.