Do a search on here...lots of posts about them or google fish soda bottle trap. Basically cut the top off a pop bottle, invert it inside the bottom. Place inside the tank with food inside and wait...Pull out when you see him go in. They get confused on how to get out.
This method has worked for me a few times
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150 mixed reef 14 fish 4 shrimp 50+ snails
55 sump long spine urchin 20 snails
Shawn