Process swai fish in a food processor until pureed and mushy; transfer to lime juice, stirring until all fish is covered in juice. Add tomatoes to fish mixture.
Blend red onion, cilantro, and serrano chile peppers in a blender until desired consistency is reached; spoon into fish mixture. Season mixture with salt and black pepper. Cover bowl with aluminum foil.
Refrigerate until fish is tender and "cooked", at least 2 hours.
