Do a series of large water changes over the next two weeks for the most natural approach, or run out and get a container of nopox and start dosing that in combination with water changes.
I am not sure the it's organics (could be) in the water column but rather just the decomposing dead matter in the rocks and sand that are making ammonia and releasing phosphates, so I would hesitate on carbon dosing until I was sure the rock and sand were cleaned.