A Priest, a Rabbi, and an Atheist Walk Into an Aquarium...

While it sounds like a set up to a really bad joke, it ends up being how a lot of people want to stock their aquariums. The biggest failing comes from the fact that fish is a giant umbrella term that doesn't even really apply to a meaningful biological grouping. 

Think of it like this: a goldfish, a molly fish and a clownfish all bear the same fishy title, but they couldn't be more diverse. Goldfish come from thousands of years of captive breeding in Eastern Asia. They come from slow moving cooler bodies of water. Molly fish come from the deltas and estuaries of Central and Southern America. They tolerate water from fresh all the way to full marine, though typically prefer low end brackish, which is more salty than fresh but not full marine. Clownfish are from the Indo-Pacific region of the world, spanning from reefs in South Eastern Asia to Indonesia and down to Australia and even Polynesia. The water they come from is salty and stable. Salinity barely changes. pH stays consistent.

I've been asked before if these fish could go in the same tank. 

Behaviorally, sure—why not?

Physiologically, they don’t even speak the same language.

Reptiles are generally understood to be ecologically diverse. Fish get put into the same category because their care looks very similar, and they all have a similar body plan. 

That's definitely not to say that mollies don't do well in freshwater tanks or that goldfish don't mind a higher temperature. There is some flexibility to each fish, but stocking a tank should be a careful consideration of each species needs. They share a label, not a world.