I've got a shoal of now four rather elderly Congo tetras. Had them since they were little and they've always been in planted tanks. I have never seen them do anything other than nibble the odd leaf when they haven't been fed over holidays or something.
They're great fish for a bigger tank, but I have to say with my three foot tanks I wouldn't buy any more - they really want more space, IMHO. 4 foot plus. Gorgeous fish though with their ragged fins.