A KH of 2 is fine for all practical purposes.
I and other's suggest 3 mainly as a large margine of error, but 2 or 1 will work if you want.
My issue is that it's not the KH, KH is not a nutrient unless it's reduced to CO2=>sugar.
Therefore harder water will have a more stable eniroment and also have more Carbon should the CO2 run low.
This is why you find better plant growth is harder waters=> more nutrients.
Plants that live in spartan environments live not because they like it, they are there because they are poor competitors elsewhere.
Many plants fall into this same situation and have similar traits. This is not just an aquatic issue.
Tonia is fairly slow growing vs Egeria najas which is very fast, but Tonia has a special niche that allows it to grow slow, but better in nutrient poor environments. It has a better root development and one could argue is therefore better adapted to poor water column conditions than Egeria.
But it does not mean it perfers it!
Non CO2 methods changes the rate, Proserpinaca grows wonderful in non CO2 systems with softer water as well, but doe sgreat in CO2 hard water systems too.
It's a nice plant, but it's not the cat's meow IMO.
Hey, you have 300 other species to grow, come back later and try it out again. I told you what I did.
Eriocaulon is a very beautiful plant. It does well and so did everything I tried out in that same tank, I had to give away buckets of B japonica, Red Cabomba, hairgrass etc.
Folks said these were all soft water plants also way back when. It's a fall back if someone cannot explain something, no one has shown me why there is a basis for it in terms of plant growth and I have yet to have found anything other than some observations/speculations, my own included over the years.
Often said, but never proved. Well, I guess I'll get to it at some point.
The research done to date suggest harder, no softer water produces better growth in several common aquatic sumbersed plants(Pondweed, Milfoil, Hydrilla).
Regards,
Tom Barr