even in a sand tank kuhli loaches would do no harm to your plants. they are just to small to do anything.
I got 3 in my tank that are some of the biggest I've ever seen almost as fat as a pencil!!!! I had to have them when I saw them for sale, they look so awesome at that size, but they only grow to be like 3" in length so never a problem with plants.
LIke the others have said, you'll NEVER see these things in a planted tank, so you might want to consider something else. They ONLY come out at night, and in a big tank are impossible to see even at night.