Well I respectfully disagree
You cannot compare what happens in a natural situation to what happens in an enclosed glass or acrylic box.
Whether it is a lake or a river or swamp, the water is naturally moving and refreshed either by rainfall, the flow of a river or a waterfall etc
You cannot fully replicate nature in an aquarium to the same effect, unless you have a massive aquarium and even those are not complete replications of nature
A filter recycles the water, a river flows thus constantly changing the water
Rivers and lakes offer a wider variety of hiding spots, plants that are significantly larger and more diverse and lakebeds/riverbeds usually consist of a mix of gravel, sand, mulm and fallen branches etc As a result fish in the wild adpat to their surroundings, a fish in an aquarium with the wrong substrate such as Cories on gravel, cannot adapt. They get hurt instead.
Mixing fish species can be done, including mixing species that you shouldn't....but mixing species or keeping hard water fish in soft water is not beneficial to their health and welfare
Having the right hardness and pH is vital for the health of fish. Just cos you can keep fish in the wrong conditions doesn't mean that you should.