Understand that the bn isn't really after the plants, it is after the algae growing on them. Even if we cant see it, it is ther., On more fragile leaves such as those of an amazon sword, the rasping of the algae is enough to rasp off the plant material too. On something with tough leaves, like an anubias or many of the other sword varieties etc., the algae is rasped of and the leaf is not damaged.
And trying the feed the bn is not going to help munch. It is in their nature to be munching away all day and night since they need lots of food going in and then they poop most of it back out. This is typical of most heavy veggie and algae eaters.