Activated carbon can make reef water look crystal clear. It can also remove toxins after a coral war. Used wrong, it can strip helpful organics and stress animals. This guide shows how to run activated carbon safely in a reef tank.
What activated carbon does in a reef tank
Activated carbon is porous media that adsorbs dissolved organics. It helps remove yellowing compounds that tint water. It also reduces odors and some chemical irritants. Many reefers notice better light penetration after a few days.
Carbon is useful after specific events. Use it after a fish death, a missed feeding, or a medication accident. It also helps after coral chemical warfare. Soft corals and leathers can release terpenes. Carbon can lower these irritants fast.
Carbon is not a substitute for good filtration. It will not fix high nitrate or phosphate alone. Keep nitrate near 2–15 ppm for mixed reefs. Keep phosphate near 0.03–0.10 ppm. Use testing to confirm trends, not guesswork.
Overuse can cause issues in low-nutrient systems. Water can get too “clean” for some corals. Pale tips and reduced polyp extension can follow. If that happens, reduce carbon amount or run it part-time. For nutrient control basics, see reef tank water parameters.
- Best uses: toxin events, yellow water, post-fragging, after heavy feeding
- Not best for: chronic algae, long-term nutrient export, poor husbandry
- Watch corals for: fading, tight polyps, sudden tissue thinning
How to run carbon: amounts, placement, and schedule
Start with a small dose to avoid shock. A safe starting point is 1 tablespoon per 10 gallons. For a 75-gallon reef, start near 7–8 tablespoons. Increase slowly over two weeks if needed. Many tanks do well at 1/4 cup per 25 gallons.
Rinse carbon until rinse water runs mostly clear. Fine dust can irritate fish gills. It can also settle on coral tissue. Use RO/DI water for rinsing when possible. Place carbon after mechanical filtration to reduce clogging.
Use a media reactor for best contact and control. Aim for gentle tumble, not boiling. Too much motion grinds carbon into dust. A filter bag in a high-flow sump baffle also works. Avoid placing bags where detritus piles up.
Replace carbon on a schedule that matches your load. For most reefs, change every 2–4 weeks. In heavy stocked tanks, change every 2 weeks. After a toxin event, change after 3–5 days. If water yellows again, it is exhausted.
- Rinse for 30–60 seconds, then run in a sock for one hour
- Use 200–400 gph through small reactors, depending on size
- Change more often if you run ozone or heavy coral foods
Troubleshooting and common mistakes
Cloudy water after adding carbon usually means dust. Stop flow and re-rinse the media. Run a 100–200 micron filter sock for 24 hours. Check that the reactor is not tumbling hard. Also check for microbubbles from the return pump.
Corals closing up can mean carbon was added too aggressively. Remove half the carbon for three days. Then restart with a smaller amount. Keep alkalinity stable during changes. Aim for 7.5–9.0 dKH for most mixed reefs.
Rising phosphate after adding carbon is a known complaint. Some low-grade carbons can leach phosphate. Choose reef-grade, acid-washed carbon when possible. Test phosphate before and 24 hours after adding. If it rises by 0.03 ppm, switch brands.
Do not mix carbon with GFO in the same reactor chamber. Flow needs differ between media types. Carbon likes gentle movement. GFO needs barely any tumble. Use separate reactors or separate bags. For more on media placement, see sump setup for reef tanks and how to use a media reactor.
- Mistake: using too much at once. Fix: start low and ramp up.
- Mistake: letting bags clog with detritus. Fix: knead and rinse weekly.
- Mistake: ignoring coral response. Fix: adjust carbon before chasing numbers.
Activated carbon is a powerful polishing tool for reef tanks. Use measured doses, good placement, and a steady change schedule. Watch coral behavior and test key parameters after changes. With a careful approach, carbon keeps water clear without stressing your reef.
Sources: Randy Holmes-Farley, “Activated Carbon,” Reefkeeping Magazine; Borneman, Eric H., Aquarium Corals; Delbeek & Sprung, The Reef Aquarium Vol. 1–3
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