Wouldn't "Do you like salty fish" be "Placesne pisces salsos?" Because to me "Placetne tibi piscis salsus" seems more like "Are salty fish liking to me" with how the declensions are used.
'Placere' means 'to be pleasing to someone'. The thing that is pleasing is in the nominative, the one(s) it is pleasing to is in the dative.
Placetne tibi piscis salsus - 'Is salty fish pleasing to you?'