Hi!! I am currently in the same boat is you... I got a Blue Tuxedo Urchin. From the moment it touched the water, it has begun devouring the Hair Algae. So far, after 2-3 weeks on some spots of rock, the algae has showed no sign of return. I have only found one downfall to the urchin... It WILL pick ANYTHING small and loose up and stick it on itself for disguise. I had it pick up a Kenya Tree frag and oddest of all it decided to remove my largest Xenia colony off its rock. After a couple hours, the Xenia was removed and the Urchin was happy munching on HA with a big Xenia on it... I would persponally suggest an urchin and that is the first time I have ever heard of a urchin removing a sessile invertabrate for disguise. (After a couple days, I removed it to no harm to the Xenia or Urchin...)
Not to get too off topic, but has anyone heard of the Katamari games?? The goal is to role up the world... The urchin reminds me perfectly of that...