Combine the flour, potato starch, baking powder, garlic powder, ginger powder, onion powder, and black pepper in a medium bowl. Combine the water, fish sauce, and sesame oil in a small bowl. Pour the water into the flour mixture and whisk until smooth. The batter should be thin.
Add the scallops, tilapia, tuna, serrano pepper, scallions, red bell pepper, and sesame seeds. Mix together with a spatula until it just comes together. Let rest, at room temperature, for 15 minutes.
Heat a medium nonstick pan over medium-high heat. Add 2 tablespoons of canola oil and allow to heat up. Take a large spoonful of the mixture and place into the pan, forming a 4 to 5 inch pancake. Cook until the bottom has turned golden brown, about 2 minutes. Flip and cook the other side until golden brown, another 2 minutes. Transfer to a paper towel lined plate to drain any excess oil. Serve immediately.