It's not a native English expression. It sounds like a translation of an idiom from another language, or something that a non-native speaker invented. The explanation that Krister has given is only found on China-based learning websites. Even the other "English" sentences on those sites look unnatural. It's better to forget these websites and find a site that is based in an English-speaking country. The only ways native English speakers use "fish in the air" is if a fish is literally flying, or to describe the smell of cooked fish. In either case, "fish" is not a verb here.

I made a mistake! How to change the best answer? - -

It's a saying.It means"a waste of time",When somebody does something, If the direction, the way wrong, they will not achieve their objectives'. For example: " You can't fish in the air " “You do it that, just like fish in the air” When someone is doing something in vain，you can say that.