Alternanthera reineckii ‘Purple’ from South America needs plenty of light to grow and form dark-red leaves. It will easily reach the water surface, and like other difficult plants growth improves considerably if CO2 is added. Shortage of micronutrients results in pale leaves. Like other stem plants Alternanthera reineckii ‘Purple’ looks best in groups. I personally would categorize this as an easy plant to maintain but difficult to have it reach its prime as in the photos.
When purchasing, especially through retail channels be aware that most are common grown emersed at nurseries. The conversion process to submersed may result leaves dropping.
Emersed growth will allow flowers to bloom.