Active (i think), my filter cartridges contain activated carbon in them.
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I am active user of GAC, mostly to minimize yellow water. It works. I have also seen examples from public aquaria that, contrary to manufacturers recommendations, GAC works months (!) to reduce organic matter. I have also witnessed that old GAC works to keep color minimized. I change GAC (2 cups per 100 gallons) every three months. There is even a scientific paper out there that suggests that in seawater, GAC may support bacteria that eats the organic that GAC absorbs, essentially keeping it active a long time. So, don’t be bashful, just use it…in a reactor is optimal.How many of you are running GAC? Actively or passively?
I haven't run any in ages but I noticed during my last water change the old water was fairly colored.

