Squeeze the juice out of the calamansi over a large bowl. Use a sieve to filter the seeds. Discard the seeds.
In the bowl with calamansi juice, add sugar, ½ teaspoon salt, and ground black pepper. Stir until the sugar and salt are diluted.
Add the ginger, chili, and onion. Stir for a few seconds until all ingredients are well blended.
Arranged the raw fish cubes in a large bowl. Pour the calamansi mixture. Toss until the fish cubes are coated. Let is stay for 10 minutes.
Toss one more time and taste. Add more salt if needed. You can also add more calamansi juice if needed. Toss to blend all the ingredients. Securely cover the bowl and place it inside the refrigerator. Let it chill for at least 3 hours.
Serve. Share and enjoy!