dark humor is a tricky thing...if someone famous dies, you generally get all kinds of jokes...e.g. "Q: what do the Kennedy's miss most about Martha's Vineyard?...A:The runway"

tasteless, but funny. but if it is somebody's parent then it is just mean. I don't know about dead fish though...i only know one dead fish joke, let me try it out...

A lady walking in her backyard looks over the fence to see her neighbors' kid digging a hole in the ground.
"Why are you digging that hole?" she asks.
"My fish died." The boy replies with tears in his eyes.
"That's an awful big hole for a fish." says the lady.
"That's because it's inside your cat."

....anything?