Urchins will consume huge amounts of coralline algae. In a nano this can cause some problems. Even a small urchin can easily clean you out of coralline in a very short time and then it will starve. I had a smallish shortspined urchin in a 55gallon tank and it had to be taken out after a few months before it ate all my algae.
I now gently poke some nori onto it and it will happily eat that. Even feeding it algae based foods will not stop it stripping your rocks bare.
Another problem is that urchins can be quite destructive. They will push themselves into the smallest gaps and dislodge any rockwork in their way. They can be pretty harsh on your corals too. Mine knocks stuff off the rocks regularly.
I would not recommend an urchin for anything smaller than a 50gal tank. Even then I personally would give it supplemental feedings.
HTH
