a big fish
An important, successful, or influential person.
重要人物
Before Donald Trump ran for president, he was already a big fish.
川普在竞选总统前就是个很成功的人了。

cold fish
Someone who shows no emotion and comes across as unfriendly or disinterested.
态度冷漠的人
I like her, but she is a bit of a cold fish.
我还挺喜欢她，但是她好像有点冷诶。
Sunny is not a cold fish at all.
Sunny 可不是个冷漠的人呢！

an odd fish／a queer fish
Someone who is very strange.
奇怪的人（queer这个单词可以指同性恋，所以不要用在不熟的人身上。）
I don't want to invite Jack to the party, he is an odd fish.
俺不想邀请强子来俺的派对，他很奇怪诶！

bigger / better fish to fry
More important matters to deal with.
有更重要的事要做
I can't worry about that now, I've got bigger fish to fry!
我现在可没功夫管那个，我忙着呢！
I want Chris to help me with this project, but he claims he has bigger fish to fry right now.
我想要Chris来帮我做这个项目，但是他声称自己很忙。

not the only fish in the sea / there are plenty of (other) fish in the sea
Plenty of other suitable persons, especially for a romantic relationship.
形容在恋爱关系中，某个人并不是世上仅有的。
When Bob walked out on Sally, all we could tell her was that he was not the only fish in the sea .
当bob离开sally时，我们只能告诉她这世界上还有很多好男人。
I know you are broken up about Janet leaving you, but there are plenty of fish in the sea.
我知道你因为Janet离开你很伤心，但是还有很多很棒的人啊！

like shooting fish in a barrel
ridiculously easy.
瓮中捉鳖，易如反掌。
Eating four burgers is like shooting fish in a barrel for Jack.
对强子来说，吃四个汉堡真的太easy了。
For Sunny, Playing mahjong and winning money at mahjong is like shooting fish in a barrel.
对Sunny来说，打麻将赢钱太简单了！