How much yummy live rock do you have? (By yummy I mean cured and mature.) You can make what are called "pod piles" with LR rubble, a place for them to live and multiply in relative safety. Clumps of macro-algae often serve the same purpose, you can see communities of bugs inside.
Do you run a refugium? This is a remote tank feeding the main tank, incorporating the elements listed above in a predator-free environment.
A good substitute for frozen brine is frozen PE Mysis shrimp, shaved into tiny bits... if your Mandarin is already taking frozen, you can hope that it switches to the much healthier mysis right away.
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Alan
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