I just noticed your thread.
I disagree with the magnesium deficiency comment. If it were that you'd see dark green leaves and paler leaf tissue. You'd also see it in other species as well not just this one.
It also doesn't really look like a potassium deficiency to me from the one picture you posted above (it might be, but it is hard to tell from a single photo). The holes aren't quite right. They are too sharply cut and irregularly shaped. They are small though, so I'd guess it is either snail damage (nerite snails?) or pleco catfish damage. Do you have either of these in your tank?
Is this problem occurring at all on any other plants? If so can you post pictures of them? Any other strange growth patterns on any of the other plants? Look at the newer growth and at the older growth. The position of the damage matters greatly if it is a nutrient problem.
You should easily be able to grow hygros with your lighting and fertilizer schedule, so don't toss them out just yet, something isn't quite right and it is killing them off, we just need to figure out what it is and remove it.