Media Reactors: How to Set Up and Get the Best From GFO and Carbon Reactors

The Upgrade From Mesh Bags That Actually Makes a Difference

Media bags in a sump work — sort of. The problem is water takes the path of least resistance, flowing around the bag rather than through it. A media reactor forces all water through the media, dramatically improving contact time and effectiveness.

What Is a Media Reactor?

A sealed cylinder with an inlet at the bottom and outlet at the top. A small pump pushes water upward through the media bed. Simple, effective, and a noticeable step up from passive media bags.

Types

• Single reactor — One chamber, one media type. Most flexible since each media is replaced independently.
• Dual reactor — Two chambers sharing one pump. Convenient but sacrifices independent flow rate control.
• Fluidised reactor — Media kept in gentle suspension by upward flow. Prevents channelling and maximises contact. Ideal for GFO and bio pellets.

UK Options by Budget

Budget (£20–40): Aqua Medic Multi Reactor, various Amazon options. Mid-range (£40–100): Two Little Fishies PhosBan 150/550, D-D Rowaphos Reactor, Deltec FR 509/616. Premium (£100+): Vertex UF-15/UF-20, Avast Marine K1/K2.

Carbon Reactor Setup

1. Rinse carbon in RODI water to remove fine dust
2. Fill the chamber about two-thirds full
3. Use a small 200–500 LPH pump (Jebao or Sicce Syncra)
4. Set moderate flow (200–400 LPH) — too fast reduces contact time
5. Replace every 3–4 weeks

GFO Reactor Setup

1. Rinse GFO in RODI water (rust-coloured dust is normal)
2. Fill about half full — start conservatively
3. Flow rate is critical: slow and steady. In fluidised reactors, the bed should expand 25–50% with gentle tumbling
4. Test phosphate every 2–3 days — target no more than 0.02–0.03 ppm drop per day
5. Replace when phosphate starts climbing again (typically 4–8 weeks)

Common Mistakes

• Overfilling with GFO — strips phosphate dangerously fast
• Ignoring flow rate — GFO breaks down over time, restricting flow
• Running reactors dry — stagnant media becomes useless
• Forgetting to replace media — exhausted media does nothing

For tanks under 100L, bags in the sump are generally fine. Above that, a reactor makes a measurable difference in water clarity and media efficiency.

Browse filtration equipment from UK sellers — media reactors are commonly available second-hand and have nothing to wear out.

What reactor setup are you running? Single, dual, or still using mesh bags?