、。
That threw me — I can't believe the hot spring inn had turned into a fish farm.
Literal
Surprised [emph-よ], hot-spring-inn [subj-が] honorific-fish [gen-の] cultivation-place [to-に] had-become [explanation-んだ] [confirmation-ね].
Typical Japanese discourse order: the reaction (ビックリした) comes first, then the explanation of what caused it, linked by the explanatory んだ. 温泉宿 'hot spring inn' is a compound typical of Japanese travel culture. お魚 with the honorific prefix お softens and domesticates the word in a child-friendly or gently polite way. になってた is the casual contraction of になっていた, where the ~ていた of a change-of-state verb captures a completed transition still observably true — 'had become (and is now in that state).'