There is a small fish-tunnel right above the tail of the whale in The Abyss. Simply swim up through it to find the Weird Fish.
Steam Achievement: Weird Fish - Discover two peculiar-looking fish...
Disclaimer: I do not own, or claim to own, any part of the displayed content - the art, music, etc...was all created by, and is owned/copyrighted by, the independent developers over at Bit Blot - the creators of Aquaria. I have no affiliation/association, or claim to have any affiliation/association with BitBlot. I am nothing more than a customer who purchased Aquaria on Steam, played it, loved it, enjoyed it, and wanted to post a play-though for Aquaria.