My Keyhole cichlid is great at making me think he's dead. When he sleeps he sleeps DEEP. Can't wake him up at all, like he's in a coma. He also changes color from brown and black to grey and black, looks very dead, especially when he's drifting around the tank rolling upside down, bumping into things and not breathing. The first time I found him like that I thought, "Awww, he's dead. Better go find the net." Took me about 5 minutes to find the net and I'd told my wife he was dead. By the time I found the net she was looking at the tank, "What the hell are you talking about? He's swimming around right there."
To keep on the topic of guppies. Those things are a lot more trouble than you'd think. I just found one dead in another tank. He'd been massively bloated for a month or two, I hadn't been able to do anything about it. He looks like his stomach burst. What a gruesome way to go. And there's another one in there that looks like he's starting to get the same way, but right now only looks kinda like he's got a double chin.