The key is the garnish ... cut up some red onions, radishes, cilantro, and lime wedges, and then add 'em to your tacos.
The sauce is also important - one part plain yogurt, one part mayonaise, and add some of your favorite salsa.
Don't use anything too fancy for the fish. I use frozen fish from Trader Joe's. The halibut is good.
Finally, the tortillas must not be dried out. You can resurrect store bought tortillas (which, frankly, tend to be dried out even when they're fresh) by nuking them in the microwave with a damp paper towel over the top for about 30 seconds.