The salad may not be clean, and the fish may not be fully cooked throughout. What works for me is to avoid stuffing the night before. A modest meal with easily digested carbs, and some fat usually works well. I'll wash it down with a cup of decaf and a lot of water. You can find a lot of variation in what works, so there is no magic bullet.
Some runners get good results from a lot of carbohydrate drink on the day before.