if you really want chiller use an ice probe or those hang on fans ( but take note evaporation will be high) however for ice probe you'd need to get an external thermostat or you put it on timer like on for 30 nmins off for 30 mins etc.
don't add anything else to your tank right now and just let it mature.. , also ... don't trust what the LFS tells you , take everything with a pinch of salt and read up , do your own research and ask fellow reefers before buying or doing anything. Marine is Not like a freshwater setup. I had an IQ5 before and because of it's basic filtration system, it's imperative you change water weekly. After all it's so small the max you'd change is 2L out of the 13L water volume.
How are you dealing with the water top up currently? Are you using Distilled water? Are you dosing bacteria at the moment?