Then he remembered Frank, and he immediately felt a surge of panic and guilt. Percy had been so worried about himself that he’d forgotten his friend was only a distant descendant of Poseidon. Frank couldn’t breathe underwater.
But where was he?
Percy turned in a full circle. Nothing. Then he glanced up. Hovering about him was a giant goldfish. Frank had turned—clothes, backpack, and all—into a koi the size of a teenaged boy.
Dude. Percy sent his thoughts through the water, the way he spoke with other sea creatures. A goldfish?
Frank’s voice came back to him: I freaked. We were talking about goldfish, so it was on my mind. Sue me.
I’m having a telepathic conversation with a giant koi, Percy said. Great. Can you turn into something more…useful?
Silence. Maybe Frank was concentrating, though it was impossible to tell, since koi don’t have many expressions.
Sorry. Frank sounded embarrassed. I’m stuck. That happens sometimes when I panic.

The Mark of Athena, Chapter XVI