Carbon and GFO: When to Replace Filter Media

Carbon

Activated carbon removes dissolved organics that yellow the water. Top Shelf Aquatics suggests about 1 tablespoon per 5–10 US gallons (19–38 L), depending on the product, starting at half and increasing over a day or two. It gives replacement intervals from 1–2 weeks to 3–4 weeks; every 3–4 weeks is a common routine.

Rinse new carbon to remove dust. Run it in a media bag in the flow, or a reactor.

GFO

GFO (granular ferric oxide) binds phosphate. Bulk Reef Supply’s instructions:

• Start with 1 tablespoon per 4 US gallons (15 L) of standard GFO, or per 8 gallons (30 L) of high-capacity GFO
• Rinse until the water runs clear
• In a reactor, keep the flow low, so the GFO barely tumbles
• Replace it when phosphate begins to rise, typically every 4–8 weeks

Don’t strip phosphate to zero

Bulk Reef Supply warns not to starve the tank of phosphate altogether. Its phosphate guide adds that a drastic drop can stress corals. Start with less GFO than the maximum, test weekly and back off as you near your target. See phosphate and zero nutrients.

Keep track

Media changes are easy to forget, and spent GFO quietly stops working. Put each on its own schedule and log phosphate weekly; a rising trend tells you it’s time.

How long does GFO last?

Bulk Reef Supply says typically 4–8 weeks, sooner with a new batch in a high-phosphate tank.

Do I need carbon in a reef tank?

Not every tank uses it. It helps keep water clear and remove organics.

Can I reuse GFO?

No. Replace it once phosphate starts to rise.