I just noticed my hygrophila plants have loads of holes and chewed up bits and transparent patches in their top leaves. It's onyl affecting the hygro (polysperma and corymbosa) and only at the top. I would think it was snail damage but, uhh, there's no snails! what on earth? i haent noticed them doing it before at all. here's a pic:
Add nutrients!!! Mine did the very same thing and I added Flourish and they turned around completely in the course of a day or two.
Edit: try this link. nutrient deficiency
here's a pic:
dunno never encountered it before with my plants 
