The "why did the red plants I received turn green" question has turned out to be one of the great mysteries of the universe. I'd like to know the definitive answer also but so far no one seems to have one. People maintain red plants with low light, high light, low nitrates, high nitrates, additional iron, no additional iron, etc. So it seems to vary a lot. I received this stem, that I'm assuming is some kind of rotala as an extra a few weeks ago. I do nothing special to maintain the color but as you can see what was one stem is now four and it's maintaing the red. I also have a bit of narrow leaf P. stellatus and it keeps a good red also. I think the key is concentrating on maintaining overall healthy plants and picking plants that are naturally red. Not on manipulating one or two parameters and not picking plants that are red as the result of such manipulation, but that's just my two cents.