Combine the dry ingredients for the tempura batter in a large bowl. Add wet ingredients and whisk lightly with a fork until everything is incorporated. If the batter seems really dry, add more seltzer. Don’t over-mix the batter.
Cut the Mahi Mahi into bite-sized pieces.
Heat oil to 350° in a deep fryer or large heavy bottomed pot. The oil needs to be deep enough for the fish pieces to “swim” and not touch the bottom. You will probably need to fry the fish pieces in batches, too, unless you have a fairly large fryer. The fish pieces should be able to move freely without touching each other. See important deep frying safety information from the USDA. Be very careful, and know how to properly extinguish a grease fire.
Do these steps for each piece of fish in the batch: Coat with batter. Carefully place in the fryer. Use a metal fork of tongs to poke it free if it sticks to the bottom
After a couple of minutes, flip the pieces. Cook until both sides are golden brown. I flipped each piece a couple of times through the cooking process.
Remove from the oil using a spider strainer and set aside onto a plate lined with paper towels.
Repeat steps for all of the fish.