Nature just means that's where those critters/plants etc are found at that point/space in time. It does NOT imply that is what is "best" for the critter/plant/fungi etc is terms of quality of life, breeding, life span, gene pool, aesthetics that we place on them etc
Aquariums, farms, livestock etc all take on their own Ecology and Genetics and these are decisively different from natural systems. While derived from nature, they are not "natural".
Many are tempted to make this assumption.
Then you get yourself in a pickle.
We have altered natural selection for our own benefit. They adapt to the new environment and we provide no competition, protection, selection of desired traits and lots of food etc.
So it's really a question that is not answered from the view point of nature, rather, what we have experienced in aquariums, the real test (how a fish responds in an aquarium to a given treatment, in this case: CO2 enrichment). From that, we can learn more about them and what are the real causes for stress and health in natural environments where we CANNOT change the CO2 and manipulate parameters practically. Lab vs field test.
You cannot argue that a lab situation is the same as a field test.
Most aquatic plants do not come from CO2 enriched waters at 30-40ppm.
But clearly, they do much much better when CO2 is added.......There are many examples that suggest "natural" systems are not what is "best".
You could make the case that all fish should not have 30-40ppm of CO2 added, not just rift fish. But they do quite well. So focusing on having harder KH and GH, not worrying about pH/CO2(unless there's too much CO2, just like for any fish) seems more applied and something that has already been demonstrated. Same for plants and soft water fish being raised in harder water and vice versa. I think it's more about having a good stable CO2 level added during rhe day, when the O2 levels are also equally higher, is the best way to look at fish health and CO2. While independent in the aquarium chemistry(CO2 and O2) , they are dependent in terms of fish respiration, and the larger the fish, the less tolerance they have for higher CO2. So high O2 all the time and some plant O2 production, and add CO2 only during the day time and keep a good tab on the fish. Non CO2 is a good goal and more like nature is many ways also. It's certainly more sustainable.
But LeftC is likely too addicted to the "CO2 dope"
I used water sprite when I lived in IN decades ago in all my cichlid tanks, floating because it had plenty of light up there and some CO2 from the air, the tap water was hard anyway. IN, OH, IL, MI etc all form what I called the "cichlid belt", lots of breeders there still today.
But all fish could be suggested to have issues with higher CO2, mostly due to respiration, the salts, remain the same whether or not you add CO2.
Adding CO2 does not alter those salts that are found in the rift lakes.