How Long Does Carbon Last in a Freshwater Fish Tank?

Adding carbon to your tank's filter helps clean the water.
Adding carbon to your tank's filter helps clean the water. (Image: Jochen Sands/Digital Vision/Getty Images)

Adding activated carbon to your tank's filtration system helps remove chemicals that can be dangerous to your fish when allowed to build up. Although carbon isn't necessary in all tanks, it's an inexpensive means of helping keep your aquarium's water clean. How long you leave it in the tank depends on factors including the carbon quality and how many fish are in the tank.

What It Does

Although it's not essential in most fish tanks, carbon helps remove chemicals such as tannins, which turn your tank water yellow. It also removes phenols, which can corrode parts of your tank equipment. It helps remove chloramine, a substance sometimes found in tap water, and some heavy metals. Many of these chemicals create odors in a tank, and using carbon keeps it smelling fresher. Carbon doesn't replace other methods of removing chemicals such as ammonia; it's an addition to your tank maintenance. You shouldn't substitute carbon treatment for regular filtration and water changes. Carbon can also filter medications from your tank, so you'll have to remove carbon if you must treat the water for fish illness.

Activated Carbon

Activated carbon comes from common materials such as wood and coal that are pressure-cooked and steamed to add tiny pores. These pores give the carbon pellets more surface area for absorbing chemicals. When using regular activated carbon in your tank, change it every three to four weeks, depending on the size of your tank and the number of fish who live there. When you have a large number of fish per gallon, you might need to change the carbon every two weeks. Watch for yellowing water or an unusual smell to the water to help you determine when it's time to make the change.

High-Grade Carbon

Although more expensive than standard activated carbon, high-grade activated carbon lasts longer in your tank. The carbon doesn't actually absorb dangerous chemical; instead, the chemicals bond with the surface of the carbon particles. High-grade carbon typically has more surface area than standard-grade, meaning the same amount can bond with more chemical particles before becoming ineffective. High-grade carbon requires changing about every three months.


If you don't remove your carbon soon after it becomes ineffective in your tank, you're not dooming your fish to death. Leaving old carbon in your tank won't cause it to release the bonded chemicals back into the water. Instead, it will simply stop absorbing new chemicals, allowing them to build up between water changes. Changing the carbon regularly helps keep the water clean, but forgetting to change it doesn't worsen your tank's condition. It's not difficult to remove; many filtration systems come equipped with empty slots designed to hold carbon. You can buy carbon pellets in plastic canisters that slide into the empty slots, or fill reusable mesh bags designed for aquarium use with carbon pellets and slide the bags into the filter slots. When it's time to make a change, pull out the old canister and replace it or empty the bag and refill it.

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