Fish tacos are pretty simple to make and almost any type of fish will work for fish tacos depending on how you prepare it. Most recipes for fish tacos say something along the lines of "use any mild-flavored, lean, flaky, white fish, like cod, haddock, or tilapia." This is helpful up to a point, but deciding which fish is the absolute best depends on what kind of fish tacos you're making, whether fried—considered traditional Baja California-style—or grilled. You can also pan-fry or even steam the fish.
It's important to keep in mind that in traditional fish tacos, which are made with mild-flavored fish, the fish itself is not the central flavor of the taco. What takes center stage are the toppings, which include strong flavors like chiles, raw onions, citrusy mayonnaise, and lime juice, as well as the flavorful batter if the fish is fried.
Fried Fish Tacos
Traditional Baja California-style fish tacos are made with battered and fried fish, usually cod or tilapia. The fish needs to be firm enough to fry well, and mild enough so that it doesn't compete with the various crunchy, creamy, and spicy ingredients that accompany the fish inside the corn tortilla.
Lean types of fish are also the best for deep frying; fatty fish like tuna or salmon can taste too oily when cooked in the deep-fryer. For a traditional-style fish taco, stick with cod or tilapia. Other choices include pollock, haddock, striped bass, snapper, and catfish, which is excellent fried.
Grilled Fish Tacos
Grilling is a great alternative to frying when preparing fish for fish tacos. It adds flavor to the fish, not only from any seasoning or marinade but also from the way it browns and crisps up on the grill.
For grilled fish tacos, you need a fish that holds up well over high heat and won't fall apart on the grill. Again, cod and tilapia are good choices, but you can also use fattier fish like tuna, salmon, and swordfish. Halibut is also a good grilling option, as it has a steak-like quality (as opposed to flaky).
Use Inexpensive Fish
Whether you are making Baja-style or not, there is no reason to buy pricy fish for fish tacos. Tacos are street food, and when filled with meat, cheaper cuts are almost always used, so why not follow suit when using fish? There are plenty of delicious, inexpensive fish options available, including tilapia and catfish.
And no matter the kind, it's a good idea to buy frozen fish. In many cases, fish is frozen right on the fishing boat and it stays frozen all the way to the market, so it is almost certainly fresher than the "fresh" fish you buy at the grocery store which was likely previously frozen, and then thawed.