Rotala Indica

Rotala indica (also known as R. rotundifolia) is an excellent aquarium plant, as it adapts to most typical aquarium water conditions. Growth is dependent on the amount of light and nutrients provided for the plant. In conditions of high light and nutrients it will grow quickly, and as it spreads across the surface of the water the plant will send down rootlets at most of the places where leaves are attached to the stem. These stems can be planted, and usually each root will grow a full stem. The more light and food provided, the redder the leaves will become, and in conditions of sufficient food and high light levels R. indica will take on a beautiful red to the newer leaves of the growing stem. The more light provided the smaller and more densely packed the leaves will be. Easy to grow, it needs to be pruned regularly, with the cut growing tips planted to replace the older stems.

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This is the only stem plant I am able to grow successfully in my low light tank. And as Guppyguy says, when they reach the top of the tank and are thus close to the lights, the leaves turn reddish.

Rotala indica is a good indicator of how much iron is available to your tank. The redder it is, the more iron you have.