A refillable cartridge will have a very visible threaded screw off top, I doubt the Coralife unit has one as they are more expensive to buy initially and the Coralife units are pretty bare bones. A new refillable cartridge is about $8 or so most places.
You might consider the custom mixed reef specific DI resins on sale here or their prefilled refillable cartridge for $18.
http://www.spectrapure.com/email/customer-...eciation.html#1You will not find better resin anywhere.