I do not think the ratio of Mg:Ca has beans to do with plant growth other than above an absolute value for each as limiting nutrient, => Liebig's law. Least that is what all the mineral nutrition experts here say (Dr Bloom, he's down the hall from me at UCD, see Epstein and Bloom for a text on mineral nutrition in higher plants) and practical experience dosing and using hard water. Ratios can build up much higher on soil particles in terrestrial systems as evaporation occurs, this is not an issue in aquatic systems.
I think Amano was referencing specifically to Alkalinity, not pH/GH.
GH has no effect and Ca/Mg are both plant nutrients and even at GH of 25 in Santa Barbara, and a KH or 11, I had little issue growing most species when using CO2.
Here's some tanks I had:
Here's L cuba growing just fien outside in the nasty tap of Davis CA, GH is 14, Mg is 50+ ppm alone...........KH is 18........
I agree the water listed is foul, I would not drink it.
I'd use an RO unit and use that.
And in general, if you can reduce the Kh to 5 or less, the easier it is for most plants to grow well.
For most aquatic weeds, hardwater(GH and KH) means more weeds, not less.
for us, it's nicer to have soft water, if we want hard water, we can simply add baking soda or GH booster.
Low KH, moderate to high GH is ideal for all species.
I cut the Davis tap with 60% RO to get this tank full of wimpy softwater species:
So it's still fairly hard.........for both KH and GH.
Results speak more than theory, but I agree in general, lower KH is better, I disagree about GH.
I think it's much more about the KH causing issues, likely with CO2 and carbon allocation in aquatic plants than uptake of specific nutrients being "blocked". Some simple test can show that they are not being blocked.
Advice: get an RO unit, drink that RO water, not the tap.
Regards,
Tom Barr