Red cabomba likely would do better with higher light than its green cousin which explains the denser growth at higher regions of the tank. It looks like its straining to reach the light. another sign is the lower leaves either withering or dropping off making the plant look very stemmy. what is yoru dosing regiment? how much FE is added?
I'm using the ferts from aquariumfertilizer.com and and dosing about 2X the amount that it says to. I do about a 70% water change every week as well. I just increased the lighting to 12 hours. If that doesn't work I may have to look into getting another light although I was told that I had med-high lighting with the fixture I have now.