Your tank really should be older. It's hard to say what size population you have now. Without giving them a place to hide and reproduce the mandarin will whipe them out pretty fast.
If you can't take it back or don't want to I would move some live rock into your sump asap and plan to build a refuge or a section for one in your sump as soon as possible.
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230 mixed reef / 230 gal sump/fuge
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