Is Carbon Bad for Fish?

Activated carbon widely serves in aquariums for chemical filtration. Its widespread use has lead to some misconceptions, both positive and negative. In most cases, activated carbon is good for aquariums, but it does have some drawbacks.

  1. What Is Activated Carbon?

    • Activate carbon is carbon that has been modified through industrial processes to be porous or full of holes. These holes increase the effective surface area of the carbon, improving its ability to perform chemical filtration. Because of this, these carbon particles absorb organic compounds from the water, improving the health of the aquarium. This same type of carbon is uses in some gas mask cartridges for the same reason.

    Downsides

    • Carbon does have some drawbacks other filter media don't. In most aquariums, biological filtration -- culturing bacteria that absorb fish waste and improve water chemistry -- trumps chemical filtration. At most, biological filter media need to be rinsed, while carbon loses its ability to perform chemical filtration and needs to be replaced. The exact time table for replacement varies from weeks to months depending on the exact carbon product. Additionally, some cheap brands of activated carbon can release phosphates. Also, fresh carbon may raise the pH of aquarium water temporarily.

    Hole in the Head?

    • Activated carbon sometimes gets blamed for hole and lateral line errosion (HLLE), also called "hole-in-the-head disease." In this disease, pits open on the heads and lateral lines of fish. The cause is not well understood, but it usually occurs to larger fish kept in crowded conditions without adequate filtration. Because of the widespread use of carbon, some aquarium hobbyists have blamed this disease on activated carbon. However, right now the leading hypothesis is that HLLE is caused by poor water quality or Hexamita parasites rather than activated carbon.

    Alternatives

    • Several alternatives exist to activate carbon. Many filters employ biological filtration -- which works better in the long run but takes longer to establish. Additionally, zeolites act very similar to activated carbon. Zeolites are silica-based minerals structurally similar to activated carbon. Unlike activated carbon, you can recharge zeolite by soaking it in a 10 percent salt solution for an hour. In saltwater aquariums, protein skimmers remove organic compounds through mechanical means.