I love my Congos, superbly beautiful fish! I only wish they'd stop being so skittish. My Congo tetras have been in my planted tanks for 2+ years and I've never seen them eat any of my plants. However, I have two points to make in this regard
and one theory.
First, they are little piggies! Big fish, always on the move, streaking about, jostling for the ladies, and they spawn almost daily. ( I know it may look like they sit still if you're close to the tank; but watch in the morning time, from the doorway or the next room.) Consequently they need sufficient food to fuel their size and adventures. I feed mine well, alternating several varieties of medium size pellet food. I try to include some spirulina pellets a few times a week. Give them their veggies.
Secondly, I also don't have any
Anubias in
their tank!

After all, isn't that why I have a third, fourth and ......... tank? Several of my other tanks are lower light w/ anubias-a-plenty. I keep the Congos in either my high light or sometimes medium light tank where the plants grow faster.
My theory: anytime I've read a complaint about Congos tetras and plant eating, it's always some
Anubias! I think as both the Congos and
Anubias are from Africa; it's just their thing. Must remind them of home. Like homebaked Christmas cookies make me especially happy and nostalgic .