hmmm... how are your nitrates? The rotala could be sucking up most of the excess nitrates in the water, leaving your swords with an N deficiency, which could appear in the form of yellowing leaves. Of course, this would more likely occur in older leaves than newer leaves. It could also be an Iron deficiency, especially if it is the brand new leaves that are yellow in color.
It won't hurt the rotala to thin it a bit... it is very hardy.
It won't hurt the rotala to thin it a bit... it is very hardy.