cardinal - It sounds like you may have Cabomba furcata, one of the most light hungry aquarium plants around. You've got plenty of light, which would be the reason the tops are staying purple but my guess is, that the bottoms are not receiving enough light, for whatever reason. To test this you can float one of the stems to see if the bottom leaves recover. I've got some of the same plant and, as long as I get enough light to bottoms of the stems, they keep their leaves, although the color is much brighter toward the tops.
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125G Planted Mbuna Tank