This is my rotala rotundifolia and when I got it out of the bag, this plant had a beautiful red hue. As it grew, the red didn't continue and I am fully aware that this plant needs high light for the red pigmentation. I don't use CO2 but I do dose pFertz weekly and excel twice a week. As I trim I noticed that the new growth, closer to the light, are slighly reddish. It's quite interesting because I notice I have a mix of reds and greens, it almost turns my rotala into a candy cane.
FYI, my light consists of 2 x 39w T5HO bulbs. I've been told that 3 bulbs maybe too much for a 55 gallon so I opted not to go one bulb more as I am truly afraid of algae outbreak.
You will see it turn redder as it gets closer to the surface. If you have no CO2, you will have a hard time getting enough light to turn it bright red without an algae outbreak.
I'm knocking on wood everyday for I have not had an outbreak yet. I used to have BBA but it's since disappeared completely with the excel. I do notice some small mats of BGA on the substrate but I get rid of them as soon as I see them.