P. clarkii are really hard on plants. When we got our first one a year ago I did some research and found that they are omnivorous, but the adults mostly eat vegetation. Ours seem to prefer plants that are starting to decay (mmm, yummy) but that doesn't stop them from shredding perfectly healthy plants.
I just noticed today that the java moss and java fern covered ornament that I dropped in their tank to give them some hiding places has been almost picked clean. They ate the java moss (I didn't think that anything ate java moss) and most of the java ferns.