Symbolism, Imagery, Allegory
The turtles and the fish show up in the fish shop down the street from Waverly's apartment… and then again on the dinner table after she's run off. Tan stresses their inability to escape as we watch them "struggling to gain footing on the slimy green-tiled sides" (6) when we first see them, and when they're being served up for dinner, the fish's "fleshy head still connected to bones swimming upstream in vain escape" (65). These little critters are super stuck.
In their entrapment, the turtles and fish parallel Waverly's own circumstances. Trapped by chess and an intensely controlling mother, Waverly eventually ends up a lot like the fish and turtles—poked and prodded and forced to comply with other people's plans—except, of course, nobody serves her up in soup. Well, not literally anyway. You might say her mom makes a meal of her childhood, though. If metaphors were your thing.