Change carbon or add to it?


New member
So I was doing some online reading again and came across someone stating that you should add more carbon on top of what is already there. My common sense says, no and it should be removed and replaced with fresh carbon.

Now, my reactor has PLENTY of room to add more carbon. Would this be a bad idea? Given the old carbon is used up, but not always fully gone. I change mine out every two weeks.

According to the BRS calculator, my water volume of 66g needs a little under 1.5 cups of carbon. The reactor I am using is the NextReef MR1 and it is currently filled about 1/3 with carbon.

So my question is, would there be any problems with running old carbon on top of new carbon?


Premium Member
I would remove the old carbon, since it is going to start functioning as a biological filter. There are several possibilities as to how good that would be, but I tend to want to use live rock for filtration.


ReefKeeping Mag staff
Premium Member
I wouldn't count on the calculator . Too many variables from tank to tank. FWIW, I use 1/3 cup of rox 8 carbon per 50 gallons of system water changed 1x per month and I 'm happy with it. In the past , for lignite and bituminous carbons I used 1/2 cup.

Once exhausted the gac might serve as a bacteria surface area with some nutrient removal but the exhausted gac will hold a lot of organics that may breakdown over time . Since the primary reason to use it is organics removal , it makes sense to me to remove it when its likely used most or all of it's capacity at some reasonable estimated point in time. I prefer rock and/or coral sand for bacterial surface areas.

Randy Holmes-Farley

Reef Chemist
Premium Member
Carbon doesn't all of a sudden one day get used up. It slowly gets less and less effective as its potential binding sites get occupied. So if you wait until it is essentially exhausted, the few days before that, it is not as effective as when first added.

There are two rationales for changing only part of it, but doing so more frequently than if you replace all of it.

The first is that there will be less likelihood of a sudden increase in water clarity when you change less more often. Such sudden increases have been known to light shock certain organisms, just like changing old bulbs.

A second reason is that if one is dosing organic carbon, one might actually want bacteria to grow on the GAC (as opposed to somewhere visible in the tank), and leaving some GAC behind to seed the new GAC a little faster might possibly be a useful procedure to keep the bacteria in the GAC canister.


ReefKeeping Mag staff
Premium Member
I agree partial removal/ replacement is a prudent practice to alleviate sudden changes. FWIW, I run gac in two locations ( about half in a canister filter and half in a reactor ) ; removal/ replacement for each of these sites is a week apart form the other. .

With organic carbon dosing I do keep several pieces of sponge material in the canister. The sponge accumulates copious bacterial mats. This seems to lessen the growth on the carbon itself or the mesh bag holding it to a point where flow is not greatly restricted when a monthly cleaning and change. is done.