Urchins that don't eat coralline?

I have a longspine black urchin that will scrape a rock clean in a day, but the good side, he also poops coraline. By the time he makes a circuit around my 90g, the coraline has regrown and he starts over.

BTW, in the process of removing the coraline, he also removes other algae from the rock. He's so efficient, I often have to drop in a piece of nori to keep my snails from starving.
 
I have 6 urchins - 2 black longspine, 1 blue tuxedo, 1 purple, 1 pink pin cushion, and a purple "impact urchin". I actually took back my first urchin because of the coralline trails, but eventually decided to try again with a small black longspine. He also seems to have increased the propagation of coralline, so that now, even with 6, I don't even see trails in the purple.
 
Tuxedo's are pretty well behaved. They will eat algae first and then coralline after that. I've had mine for 2+ years and my coralline is still intact.
 
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