Biological Filtration in Reef Tanks: Live Rock, Ceramic Media, and the Nitrogen Cycle

The Filtration You Can't See

Biological filtration is bacteria — trillions of organisms on every surface, converting toxic waste into less toxic waste. No gadget to buy, no reactor to set up. It's the single most important filtration process in a reef tank, and it's entirely invisible.

The Nitrogen Cycle

Ammonia (NH3/NH4+): Fish waste and decaying matter release ammonia — extremely toxic even at 0.25 ppm. Nitrite (NO2): Nitrifying bacteria convert ammonia to nitrite — still toxic. Nitrate (NO3): A second bacterial group converts nitrite to nitrate — far less toxic and actually beneficial in small amounts for coral growth. Denitrification: In low-oxygen zones deep within porous rock, bacteria convert nitrate to nitrogen gas, removing it from the system entirely.

Live Rock: The Original Biological Filter

Porous rock colonised by bacteria, coralline algae, sponges, and micro-organisms. Outer surfaces support aerobic nitrification while deep interiors create anaerobic zones for denitrification.

How much? For a 200L tank, 15–25kg of quality porous rock is a solid starting point. Highly porous options (Real Reef, CaribSea LifeRock) provide more surface area per kilo.

UK options: Dry rock (£8–15/kg, pest-free but needs seeding), ocean rock (£3–5/kg, less porous), aquacultured live rock (pre-loaded with bacteria), or second-hand live rock from UK sellers.

Ceramic and Synthetic Bio Media

Additional bio media in sumps provides extra capacity. Popular options include Marine Pure blocks (£30–50), Seachem Matrix (£10–15/L), Siporax sintered glass (~£15–25), and Brightwell Xport. Place in a sump chamber with moderate flow — no reactor needed.

Cycling a New Tank

1. Set up rock, sand, saltwater, and circulation
2. Add ammonia source (raw prawn, fish food, or Dr Tim's Ammonium Chloride) to ~2 ppm
3. Test every few days — ammonia spikes, then nitrite, then both drop to zero
4. Do a large water change to reduce nitrate, then add livestock

Timeline: 4–8 weeks with dry rock. Faster (2–4 weeks) with mature live rock, established media, or bacterial supplements like Dr Tim's One and Only.

Protecting Biological Filtration

• Medication — Copper and antibiotics kill beneficial bacteria. Always use a separate QT tank
• Power outages — Bacteria need oxygenated flow. A battery-powered air pump (~£10) is essential backup
• Over-cleaning — Never rinse bio media in tap water (chlorine kills bacteria). Use tank water if cleaning is necessary
• Temperature/salinity swings — Shock and kill bacterial populations

Browse filtration equipment from UK sellers — live rock, ceramic media, and complete filtration setups are often available from fellow reefers.

How did your last cycle go? Any tips for speeding things up?