That's a hard one actually. See I like sushi, So It really depends On two things .
First; How fresh the Fish is, and Secondly, How well do you want It cooked.
With most Fish, I've found It's about the Flavor, However, IF your making say some Catfish in Cornmeal Breading, Just fry It untill the breading is Compfortubly Brown for you.
As far as a slow Boil, I usually interpret this as Abit more then a simmer, but less then Solid Water vapor coming off the pot. An example, I like my pasta El Dente, wich is kinda Firm. when fixing for my family, I usually Cook the pasta Very slow, and pull my self a bowl out , and then turn up the heat, so they can eat there Mushy Pasta. But It varies on what you are cooking. I can Cook Pasta in an electric skillet at 300 degrees for 5 minutes, and Get It like I want It, or kick It up to 350 -400 Degrees, and Make It soft and tender in about 10 minutes.
Fish is probably one of the hardest things to cook for another person though, as everyone has there own taste on that wild, Varied animal.
Stephen