I've found my amanos to be aggressive eaters and highly food-motivated. I've seen them snag food out of the water column, and it's hilarious to watch a shrimp steal a huge piece of flake, run off with it and munch away. You can almost see the smug look on their face.
If you have fish in your tank I'd suggest to overfeed just a little so some reaches the bottom for the amanos to munch on. Generally I'd say they probably aren't eating live plants, but as mentioned above if food is in short supply who knows.