While it is okay to marinate your fish overnight, it is not the best method. The fresher the ceviche, the better.  When preparing this recipe, the yellowtail ceviche will be at its prime after marinating for 30 minutes to one hour. This gives the acid enough time to “cook” the fish and infuse it with flavor. After about 2 hours, the fish will begin to change in texture as it breaks down. Serve your ceviche tostada as fresh as possible, but don’t fret if marinates a bit longer. Stored in the fridge, in an airtight container, your yellowtail ceviche is safe to eat for up to 2 days.