Why are my fish swimming at the top of the tank water?

My fish just smacked the top of the water, then sunk to the bottom and laid flat. After 20 seconds he shot back up and is swimming fine. Any idea what this

Why are my fishes swimming at the top? They've never done this before and my guppy just passed this morning and now I'm worried : r/Aquariums

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Why Is My Gold Fish Keep Swimming On Top Of The Tank ?

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