The species of Alternanthera are not plants strictly requiring very soft water according to my past experience, maybe a lot of aquarists will say that the water of medium KH(5 to 8) is ideal for them, yet some of them can grow very well in the soft water of 1
KH or lower, and that is the case I post here, which is contrary to what I read from some Japanese literatures. Therefore, I think the answer could be very complicated, furtheremore factors should be considered.
To make it more reddish, I use 5 fluorescence lamps of 40W to supply the light for my tank(120x50x67), and an overfertilizing DIY Co2 system to accelerate its growth. Meanwhile, one thing very important for the species of Alternanthera is that they usually stop growing when the temp is higher than 30, therefore a cooler is necessary to keep them especially in summer.