Hi. I would invest in a refractometer to test the salinity since the tank is so small, any swing can be deadly. They are way more accurate than the swing arm meter and cost about $30 on ebay. I have this one
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I'd say make sure you have a liquid test kit like the API saltewater master kit, about $20 on amazon.com.
You might want to find a tiny powerhead if the filter does not supply enough flow.
Thats about it as far as i can think of off the top of my head. Just add live rock and some sand and you should be set.