Use your favorite type of chili powder. We like chipotle chili powder, but this will definitely add some heat. If you're looking for low to no spicy heat, then use regular chili powder.
If you can't find mahi-mahi, you can use quality tuna steak, or swordfish, or cod.
We highly recommend placing the marinating fish into a grill pan, or a cast iron skillet before grilling. Alternatively, you could can thread the fish pieces onto skewers.
If warming the tortillas on the grill, don't leave them on too long, otherwise the heat will dry them out. You can place them in a foil packet and warm them that way. You won't get the nice charred grill marks, but they won't dry out that way.