I suppose that some would apply to tropicals. Although JoAnn (the Goldfish guru) wrote the article specifically in reference to Goldfish.
There are some major differences in a regular fish's gut vs a fancy goldfish. The fancy goldfish's gut is very much shorter and squashed due to breeding selection for the body shape, so they have special considerations as far as feeding & pooping goes. Eg. A goldfish's poop comes out almost immediately (maybe a few minutes) after eating. So what you see in the poop reflects the current meal. Whereas in tropicals, the poop might be a result of foods eaten days ago.
Also, the appearance of egg sac & reabsorbed egg is pretty goldfish specific.
However, things like thin white poop being indicative of internal infection (or parasites) or bloody diarrhea being bad is pretty universal.