In that thread it was being discussed about some odd ball type of problems that one of the members here was dealing with. I also at that time was having some similar type syptoms with my plants in respect to amounts of Ca and Mg in the water. Basically I still do not know why or for what reason I would expierence the syptoms I did which basically look like a Ca deficiency, but I do know when I add a bit of seachem eq, and epsom salts to my tank at water change these syptoms go away, even though my tap water has plenty of Ca in it already. Some thought that it was POSSIBLE that it had to do with Ca and Mg ratio's and levels along with the KH of the water.
I really cant say for sure for me what caused these things, TW might have a good reason for you, if he was able to slove his problems. But in these odd cases it was thought the the KH.... once again i stress this.... POSSIBLY may have had some sort of an affect on the plants abality to get Ca Mg or both....
Really im no expert on it i really dont know, but basically in regards to your question..... no the idea that plants generally do well, possibly better in harder KH waters has not changed, I dont know what the "ideal" KH range would be, but im sure others are going to chime in on this so I will leave the scientific stuff up to them
No, KH refers to carbonate hardeness.... Calcium and epsom salts (a source for Mg also known as MgSO4*7H2O) are the two main things that make up the general hardness or GH. Ca and Mg both affect the GH of water, but in no way affect the KH of the water.
Yepp, there are several different sources for Ca, and depending on the situation you would pick which ever would fit best. Calcium carbonate would raise GH and KH, and on the other hand Calcium sulfate would just affect the GH. Just need to pick which ever fits I suppose.