What does "mad fish" mean in Bring The Pain by Method Man?

Would likely be slang/word play on "very suspect" as the line refers to explaining just how deep shit can get, not literally but metaphorically with double meaning.

The adjective "mad" is commonly used black American slang meaning "many," (as in "I got mad stacks of cash," meaning "I've got a ton of money"). Therefore, one likely read is "many fish."

However, let's look at it in context:

Check it, just how deep can s* get
Deep as the abyss, and brothers is mad fish accept it

The most direct read of this is that "brothers is [are] mad" because "fish accept" Method Man's deep thoughts.

So why fish? "Fish" has a number of slang meanings, but in this case, I think it's just a play off the immediately preceding invocation of "the abyss," a deep underwater cavern.

Method Man is as intellectually deep as the abyss. Jealous people are petty enough to be mad that even under the ocean, his rhymes are acclaimed.