if you really like the urchins, you could possibly look into a different kind, there are many out there. Pincushion is one common type. I don't have any experience with them but I have heard that they can get quite large and sometimes knock over rocks. I have a blue tuxedo that I just got and he is really tiny, so even if he eats coralline, he's in a 75g so I don't think he'll hava a chance to eat *all* of it. Some people have said yes, they eat all your coralline, others have never noticed. One guy even told me that his really loves hair algae. It is really up to you, if you love how they look, it might be worth it. But I would reccomend looking into a blue tux.