If you want to hyphenate the word, the correct spelling is "Fisch-Sandwich" since you connect two nouns. In German, usage of the hyphen is rather free and to some part a matter of taste and style. If it connects two nouns, it is mainly used to 1) emphasize a component of the word 2) to disambiguate between different interpretations of the word 3) to make long combinations of words more readable. You can find some examples here: http://www.duden.de/sprachwissen/rechtschreibregeln/bindestrich#K21 As a rule of thumb: don't use it unless you have a special reason to do so. In the case of 'Fischsandwich' I wouldn't use it.