There are tons of recipes for this, and no two are the same.
Unlike traditional ceviche in which fresh fish is "cooked" in lime for hours, ceviche Nikkei includes fresh fish exposed to lime for only minutes before eating. Nikkei cuisine refers to Japanese-Peruvian food, first created after Japanese farmers immigrated at the end of the 19th century to Peru---the first country in South American to have diplomatic relations with Japan. Nikkei cuisine centers on the use of seafood---fish, octopus, scallops---and integrates Japanese ingredients like miso, ginger, soy, and wasabi with Peruvian staples including aji pepper, potatoes, and corn.
You can replace the tuna for a white, mild fish like sea bass or sole, and even salmon.