A piece of cake - like shooting fish in the barrel

Example

I'll show you how to get to the top of the mountain. Don't worry, it's a piece of cake

To smell something fishy

Example

Matt's statements are rather ambiguous, I smell something fishy going on

A fish out of water

Example

I do not feel comfortable here. I am like a fish out of water.

Neither fish nor fowl

Example

That house in sale has an architecture neither fish nor fowl

Like a fish out of water

Example

I do not feel comfortable here. I feel like a fish out of water.

Another (a different) kettle of fish

Example

As soon as you start writing code to update a database, it's quite another kettle of fish

Like a fish out of water

Example

I want to go away, I feel like a fish out of water

The lion's share

Example

I had no doubt that Darryl would take the lion's share of the company after his father's death

In the lion's den

Example

Be careful in that neighborhood you may end up in the lion's den

Be like a dog with two tails

Example

Jane will be like a dog with two tails if she gets into dancing school with her sister

There are plenty of more fishes in the sea

Example

Don't worry about it, there are plenty more fish in the sea, you will have other opportunities

Bird brain - pigeon brain

Example

Some tv shows gather bird brain viewers