I don't want to put words into elegance coral's mouth, but I think when he said "tough" he meant that they are pretty hardy, once settled in. And I have to agree with that. (( assuming that you are talking about a Haddoni carpet )).
I currently have 3 ( including a green one ), with my oldest one being over 9+ years old. They have lived through a handful of mistakes on my part. But, the hardest part of this is finding a healthy one, my green one is my 3rd green one, the other two didn't last a week, never settled in. Not to sound cocky, but I know it wasn't my fault that those two didn't make it -- they were sharing a tank with a blue one that never acted different while they were wasting away.