User blog comment:SayuriDarling/The House of Hades Official Discussion/@comment-4503464-20131107130909/@comment-5927468-20131110003541

Maybe it was still kind of water at that point, so Percy could still control it. As for controlling poison in HoH. I kind of got the impression that he could only do that in Tartarus. Before he tries, Percy even reasons that he should be able to because rules are different there, citing the fact that there was a river of fire, monsters rising from zits, and all that crazy stuff.