They have a taste for certain plant species. They love most rotalas too. I have a HUGE clump of rotundiflora in the back of a heavily planted 180G, and over the course of a year or so, most of the back side is nibbled away. What's grown closest to the waterline is left alone. So location is a consideration as well with adults. Other than that clump of rotala stems, I've stuck to swords, crypts, and anubias. Recently I added a japanese spatterdock and they've left that alone. On the low side, they haven't touched the pygmy chain swords. They also leave H. Corymbosa alone for the most part but will nibble the tips of angustifolia until square.
I keep potted plants in all my breeding and rearing tanks and they are all limited to crypts or swords.
Somewhat related: really small (sub-dime) angels will decimate any duckweed infestation, and fast. They nibble off the plant root completely.
"Why can't my wife see all of this stuff as an investment?"