The use of a Carbon source in a marine aquarium is the result of an old technique to lower strongly and quickly the levels of nitrite, nitrate, and phosphate present in the water.
This is like the technique of vodka/vinegar/sugar dosing, but without the risk of introducing unwanted elements (coloring, flavoring, etc.) and with multiple carbon sources in right proportions.
The added organic carbon strongly stimulates bacteria and quickly lowers the levels of NO2, NO3 and PO4. This bacterial surplus will be consumed by the animals in the tank or eliminated via efficient skimming.

[N,P]EX is a combination in the right proportions of several different carbon sources which guarantee its effectiveness. It should only be used by aquarists who are experienced and knowledgeable in monitoring the values of their aquarium.

It's recommendend to use when all other NO3 and PO4 limitation measures are no longer effective. Gradually increase the amounts according to the expected parameters levels. Once they are stable, maintain the dosage while regularly monitoring these levels.