I am not Diana Walstad.
Here is the reasoning behind keeping the GH and KH sort of close to each other:
There is not really a direct connection between GH and pH, but the indirect connection is this: The minerals that we read as GH are Ca and Mg. In nature they are most common as calcium carbonates and magnesium carbonates. Where you find lots of Ca or Mg in the water, you also find lots of carbonates or bicarbonates.
In areas of the world with lots of limestone and related materials, low rainfall, high evaporation rates (usually a combination of all 3) the rain dissolves the minerals and the water is high in both GH and KH.
In areas of the world with LOTS AND LOTS of rain, the minerals have:
a) Been washed away a long time ago, leaving only minerals that do not dissolve very much. The GH and KH in the water is very low because not a lot of limestone or related materials are still there to be dissolved.
b) Are still dissolving, but are so highly diluted by the copious rain that the GH and KH in the water is very low.
In either case the GH and KH are both very low.
There are other spots, where GH and KH are nowhere near similar to each other, but these are so few, and so small, that most people do not keep fish from those areas.
Lets return to the case where there is little rainfall, high levels of limestone or related minerals, and high evaporation. This sort of environment is not friendly to plants, so the land surrounding the lakes and rivers does not drop a lot of organic matter into the water. Not much runs off the land because there is not much growth, there is not much rain, what rain there is soaks into the soil instead of running off.
Fish from that sort of water are acclimated by evolution to hard, alkaline water with high TDS.
Now lets return to the high rainfall area.
This area is lush with plant growth. Plants drop leaves and branches, and whole trees onto the forest floor, and into the rivers. The rivers may flood out onto the forest floor. Remember these waters have very low levels of minerals and are now being exposed to high levels of organic matter. Organic matter tends to add acids to the water. Several organic acids, and the acid reaction of decomposition. When the water is so low in KH, the end result is very acidic water.
-----------------------------------------------------------------------------------------
So, take your pick:
Fish that have evolved in soft (low GH) water are also acclimated to acidic water (low pH). That is the way the water is in their habitats.
Fish that have evolved in hard (high GH) water are also acclimated to alkaline water (high pH). That is the way the water is in their habitats.
Sure there is a middle ground. GH and KH in moderate levels, and pH not too far off neutral.
However, this post is specifically about the plants and animals that are from very soft water that is acidic.
The way I have made this sort of water is to start with RO and blend some tap water to get just a few minerals, setting the GH and KH about 2 German degrees of hardness. Then filter that through peat, or add oak leaves and twigs, or driftwood. Something organic to add the organic acids.
In other words, set the GH to the optimum level for the fish. Other people have figured out that the GH is the most important factor for keeping fish healthy. There are a few plants that are also very picky about their water, and these plants do not thrive with high calcium in the water.
Then set the KH about the same, because that is what you find in nature.
By setting the KH similar to the GH the pH will usually follow: Acidic in the soft water tanks, alkaline in the hard water tanks.
----------------------------------------------------------------------------------------------------------------------
Here is one report about certain lakes in temperate zones. Scroll to page 303 (about the 5th page down) and see the chart graphing Ca and Mg vertically and alkalinity horizontally. See how the values tend to cluster in the middle line?
http://aslo.info/lo/toc/vol_3/issue_3/0299.pdf