I had issues with my val when I first got it, but at that point I was a novice. I wasn't injecting CO2 and I wasn't adding ferts to the tank. I also have red foxtail in my tank right now, along with a bunch of other very fast growing plants (e.g., cabomba, wisteria, ludwigia) and I doubled the dose of ferts I have been putting in since adding those plants. It really is amazing how quickly foxtail grows. I think it grows about 6 inches in a week under my current tank conditions.
Are you using any gravel fertilizer like Flourish plant tabs? What kind of substrate? If I had to guess on something, I would guess that some nutrient is being sucked out of the water by the foxtail and there just isn't enough left for the val. It might even be CO2.
You might need to do some testing of the water parameters to see what might be lacking. My parents and siblings think I'm the mad scientist when I am doing water testing for my tanks, but I think it really is needed to grow healthy plants and fish.
