There is little evidence that nutrient uptake in plants is easier in soft water than in hard water. That has to be the product of someones dream. If anything, the nutrient uptake rate typically increases with increasing hardness. There are two basic mechanisms of nutrient uptake in aquatic plants:
The first is a passive mode where the concentration of the nutrient ion exerts an osmotic pressure against the leaf. The higher the concentration, the higher the pressure. The osmotic pressure of a nutrient ion is completely independent of the other ions in the water. Therefore, it doesn't matter whether the water is hard or soft. 10ppm NO3- is 10ppm NO3- regardless of whether that NO3 is in RO water or in water with KH 20.
The second mode is an active mode where there are specific enzymes dedicated to attract and hold that nutrient molecule. The enzymes are embedded in the outer membrane of the cell and are usually capable of distinguishing between their target ion versus random ions, although there may be some exceptions. So, even in hard water which has an abundance of ions, the enzymes capture the nutrient ions of interest and allow them to pass through the outer membrane layers unimpeded. A higher ionic content in the water actually pushes the target nutrient ions away towards the receptor enzymes.
Then of course there are the roots, which have similar electrolytic processes for uptake.
So no one actually knows why Tonina has difficulty in hard water because we don't know what we are measuring, and it could easily be that Tonina has a negative response to some compound which is typically found in hard water but which may actually have nothing to do with hardness, or it may be responding to only a specific characteristic of the hardness.
Also, KH really is not the same as hardness, so this causes even more confusion. Your KH test kit doesn't actually measure the Carbonate content of the water. It measures the water's ability to sequester/neutralize acid (H+), which, Carbonate and bicarbonate are famous for, but which many other ionic species (Na+, K+, Ca2+, Mg2+, Cl-, SO42-, and NO3-) are also quite capable of to a greater or lesser extent depending on their concentration within the water column and depending on their interaction with other ion species . The ability of water to sequester acid is referred to as alkalinity. So your KH test kit measures alkalinity because it cannot identify carbonate or bicarbonate from the myriad of other ions in the water which affect the behavior of acid.
That's why there is so much variation in response characteristics in different tanks. Two people can measure KH to be exactly the same in different locations, one tank has more Carbonate/Bicarbonate than the other. If the effect on a fish or plant is related to the level of Carbonates then there needs to be some other measuring device to determine what the actual Carbonate level in the tank is in order to make a rational assessment. If the fish or plant is sensitive to one of the other negatively charged ions that affect alkalinity you will not know what that ion is from a KH test kit.
There is a similar ambiguity with GH test kits because they can NOT differentiate between Magnesium and Calcium. So if a fish or plant has a negative response to Calcium, but not to Magnesium, you need more specific equipment to determine what portion of the GH Calcium contributes to. These are yet more reasons why hobby grade test kits suck.
Cheers,