Fish: Make sure to use at least one fish that has some gelatin in it. I use rockfish from the Pacific Northwest to give the soup the body it needs.
Rouille: There are a million variations of rouille. The version listed below will give you the same flavors found at most restaurants serving rouille. Remember, people eat results — not methods. I sometimes add some boiled potato for adding body. In the end, whisk in a touch of your fish soup to give more flavor to your rouille. Make a big batch of rouille and try eating it with almost everything you can imagine — vegetables, cold roast pork, chicken, on sandwiches.