Powdered sugar is a super fine sugar, finer than even castor sugar. It is often termed as 10X sugar, confectioner's sugar or icing sugar. In France, it is called Sucre Glace. The size of the sugar crystal varies 10X sugar is usually 0.010 mm, while confectioner's sugar is 0.060 mm, and icing sugar is 0.024 mm.t is a type of granulated sugar that has superfine granulated crystals, usually 0.35 mm. The term castor or caster sugar is mainly used in UK English and its Commonwealth countries. In North America, it is sold as super fine sugar.
Standard baking temperature is 350 degrees, which works just fine for fish. But you can get away with setting the temperature even lower. This extends the cooking time, which may help if other ingredients in the dish need more time to cook.
Fish has always had one temperature for being done. The internal temperature for a done fish should be 145 degrees. At this temperature the fish should be opaque throughout and flake easily when tested with a fork. Like you, I prefer my Salmon to be medium, so the internal temperature will not be 145. As far as the internal temperature for other fish. I think you will need to test some fish to find the right internal temperature for the way you like your fish cooked. I know that on Cod and Halibut, I under cook mine a little so it is moister.