The picture below was posted on facebook and received many replies that identified possible problems with the water parameters and/or nutrients. The well camouflaged, and happily feasting Amano shrimp was largely ignored!
Amano shrimp usually graze on damaged & deteriorating leaves. However, many can and well eat certain plants even when they are healthy - This list includes Rotala wallichi, Alternanthera reineckii, Downoi amongst others.
A 3-point checklist to identify herbivore damage
Look out for (1) "Cut-out" patterns: these are usually made by teeth / claws, especially on the (2) interior of (3) healthy leaves. When healthy looking leaves have 'rough-cut holes' not at the edges but on the inside, it is often a clue that herbivores have been busy.
Classic example of herbivore damage (picture shows a shrimp, but the trail pattern of the holes indicate its likely to have been done by a large snail)