Purigen and carbon are completely different and achieve different objectives. Purigen is a nitrogen scrubber so to speak, it reduces the organics in the water and aids in improving the water quality. Carbon works a little along those lines but not as efficiently or long term. Carbon on the otherhand removes a great many things both good and bad but more important, the bad. There are a host of things that are in the aquarium that you cannot see or test for that carbon will remove and improve the quality of life for your inhabitants. This is especially importanty in reef tanks where it removes coral toxins, improves light quality/penetration and other toxins.
If nitrogens are a concern, work on improving the export of the system as well as examining stock levels/feeding routines and overall set up. Carbon IME
, is good to use no matter the tank and employed 24/7.