This is a grand tradition here in San Antonio. You use a firm white fish such as cod, basa or pollock, cut in fairly large cubes; tilapia and catfish, not so good.
Some people batter fry the fish. Around here we grill it. You will season, grill, put on a heated corn tortilla; top with a vinaigrette type slaw and chimichurri sauce.
Here is a short video on chimichurri sauce:
And here is a recipe:
I allow 4 ounces per two taco serving. You probably won't have more than 6 try it the first time, except now during Lent more people look for seafood, especially on Fridays.