Is there any live rock in it that isnt cured? If the rock is cured the water is old and the filter is old then there will be almost no cycle. A small fish should be ok in my opinion. As for the refuge if it isnt acting as a sump as well i would say fill it to the top, more water volume. But if its a sump you have to remember what will syphon down into it in the event of a power outage.
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Sean
Back in the good ole days 
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