What is correct: 'the reason why' or 'the reason that'?

[Wind]

The reason (?) fish is so good for the brain is the so-called omega-3 fatty acids (??) it contains.

1.Is there should be why or that in (?)
2.Is there should be which or that in (??)

Thanks

[Spencer]

Hey Wind,
I'd write why, and which, if I were You.
( I might be wrong though)
Spencer

[Wind]

Hi Alan,
So what is the answer?

[Conchita]

Hello Wind,

Spencer has already given you a correct answer.

Still, you can also leave out the pronouns in this sentence:

The reason fish is so good for the brain is the so-called omega-3 fatty acids it contains.

Note one correct way to ask your questions:

Should I use 'why' or 'that' in...?

[Wind]

Hi Conchita,

Thank you very much.