I use serranos because they connect this version to classic aguachile verde: the same chile and lime, but charred until blackened. Pasilla adds a deeper, toastier, almost chocolatey flavor and gives the sauce a little body without making it heavy. Some cooks blend in a blackened tortilla, which is delicious, but I prefer my tostadas alongside the aguachile, not in the salsa.
You can use almost any chile you like, including jalapeño, habanero, serrano, or even a little blackened poblano if you want less heat. Soy sauce provides salt and savory depth. Many cooks in Mazatlán also add Worcestershire sauce or Maggi. I like Worcestershire sauce, just not in Mexican food, so I leave it out. That is entirely personal preference. Use what tastes good to you.
