Mix the dark brown sugar and fish sauce thoroughly, optionally adding the red pepper or chili to help with less-pleasant fishiness. Marinate your fish of choice for 2-3 hours, depending on the size and variety of your fish. The longer you marinate, the more the fish "juices" will be drawn out by the high salt content of the marinade. This will give you a saucier dish, but can potentially alter the taste and texture of your fish. For our chilean sea bass, we go less than 2 hours.