Yellow leaf - should I "leaf" it alone?

[Bioworldmaker]

I was just staring at my plants...
I have 2 large amazon swords, 2 red ludwigia and 1 foxtail.
One of the amazon swords has leaves turning yellow. Now I wonder why that is happening. My other Amazon sword is lush green gorgeous, my red ludwigias are wonderful, my foxtail... uh... goes so-so. I also have some brown algae, I read somewhere they come from tap water or something like that.
Why is that leaf turning yellow? All of the others are fine. I wonder, if it were lack of nutrient/light then the other amazon sword would have the same problem?
Also, should I remove it or just leave it alone?
Thankyou thankyou

[Bearfan]

What kind of lights? Any CO? What size tank?

[medge00]

and are you fertilizing at all?

[trenac]

Amazon swords are heavy root feeders, so stick a root tab under it and see if that helps.