Luckily we don't know how Aqua will figure it out. I just hope he will realise already in the next episode that simply believing what his half-brother said, a half-brother he doesn't really know to any degree at all, might not necessarily be true. I'd prefer that way to some grand scene where somebody else will say it to him, in any manner, opening his eyes. After all, it would be nice if he had his own wits, despite this momentary lapse. Though I suppose I could accept Aqua getting the general hint from a detective story. The Japanese police is infamous for being incompetent, but they must still have detective stories.