Thought you may have one with a liking for plants, most will not eat / tear up plants. Though, I have seen stranger things happen.
Try adding some anubias which have harder leaves and are very easy to keep. Also if he/she decides it still wants to eat the softer plants it probably that he is laking something in his diet. Most do accept flake foods and some live foods. Depending on other tank mates also because if there are bigger more aggressive tank mates they may be keeping the shark from getting his share. In this case you may have to assure that the shark gets a fair share.
What are his/her tank mates?
Is there any other sharks in the tank?
If the problem persist you may need to add some low cost plants with soft leaves which you can replace as needed.
For the most part some fish have tendecies to distroy even if the are not eating it.
Though keep in mind if the fish is happy and eating well DON"T disrupt his eating habbits as to this may cause him to stop eating all together.
Years ago I had 2 a red tail and a all black and both nibbled on the plants yet didn't distroy them.
Keep in mind these are bottom feeders and LOVE Tuiflex worms. Though be VERY carefull what you feed because some species of tropicals don't do well with live foods.
What are the tank mates
Hope this tid bit helps.
feels wierd since I've been away for so long so bare with me here.
Danny