On this page
Rock is the foundation of a reef aquarium. It's where most of your beneficial bacteria live, where corals are placed and where fish shelter. It also shapes how your tank looks for years to come, so it's worth choosing carefully. The first decision is whether to start with live rock, dry rock or a synthetic rock.
The options compared
| Live rock | Dry rock | Synthetic or ceramic rock | |
|---|---|---|---|
| What it is | Rock taken from or kept in the ocean, carrying bacteria and life | Natural or quarried rock with no life on it | Man-made rock designed to look natural |
| Cycle speed | Fastest | Slower; needs seeding with bacteria | Slower; needs seeding |
| Biodiversity | High: coralline algae, sponges, pods and worms | None at first; builds over time | None at first |
| Pest risk | Higher: aiptasia, crabs, mantis shrimp | None | None |
| Cost | Highest | Lower | Varies |
| Best for | Experienced keepers who want life from day one | Most new reef tanks | Custom shapes and eco-conscious builds |
Why most new reefs now start with dry rock
Dry rock has become the most popular choice for new tanks. It carries no pests, it's cheaper, it's easy to shape and glue, and you can take your time getting the aquascape right before adding water. The trade-off is a slower cycle and a period known as the "ugly phase", when diatoms and algae appear before the tank matures. With patience and a good bacteria product, it's very manageable.
Popular dry rocks include Dry Ocean Reef Rock, Caribsea South Seas Base Rock and Caribsea Life Rock, which is pre-seeded with dormant bacteria and coloured to look more mature. For something more natural, Real Reef Rock is grown in saltwater on land and has a coralline-like finish. TMC EcoReef Rock is a ceramic option that's easy to build with. Browse our full marine rock range.
How much rock do you need?
A common starting point is around 0.5–0.75kg of rock per 10 litres of tank volume, but this depends on your aquascape and the rock's density. Modern reefs tend to use less rock than older ones, leaving more open water for fish and more flow around corals. Porous rocks such as Real Reef or Life Rock weigh less for the same volume. If you're not sure, buy slightly less than you think and add more later.
Planning your aquascape
- Build outside the tank first. Lay out the rock on a towel or a cardboard template the size of the tank base.
- Leave space for flow. Keep rock a few centimetres away from the glass so water can circulate behind it and you can clean.
- Use open structures. Arches, bridges and islands give caves for fish and many places for corals at different light levels.
- Place rock on the glass, not on sand, so burrowing fish can't undermine it.
- Keep it stable. Nothing should rock or wobble. A collapse can crack glass and crush corals.
For larger builds, bond pieces together with an aquascaping cement such as Nyos Reef Cement, or use Seachem Reef Glue for smaller joins. Some aquascapers also pin pieces with acrylic or fibreglass rods. Read our marine decoration guide for design ideas.
Choosing a sand bed
Most reefs use a 2–4cm layer of aragonite sand. Live sand such as Nature's Ocean Bio-Activ Reef Live Sand or Caribsea Ocean Direct Oolite adds bacteria to help the cycle. Finer sand suits gobies and sand-sifting snails; coarser sand is less likely to blow around in strong flow. Browse live sands and dry marine sand.
Preparing and curing rock
Dry rock
- Rinse it in fresh water to remove dust.
- Some dry rock, especially rock that has been in the ocean, can release phosphate. Soaking it in saltwater for a few weeks and testing phosphate before adding corals avoids problems later.
- Seed it with a live nitrifying bacteria product such as Dr Tim's One & Only or Fritz TurboStart 900.
- Give the bacteria an ammonia source to grow on. A measured dose of Dr Tim's Ammonium Chloride is cleaner and more precise than adding fish food.
Live rock
Live rock that has been out of water for any length of time will have some die-off, which releases ammonia. "Curing" means keeping it in heated, circulated saltwater with regular water changes until ammonia and nitrite read zero, usually two to four weeks. Inspect each piece for pests before it goes in your display. See our guide on reef pests.
Cycling your new reef
Whatever rock you choose, test ammonia, nitrite and nitrate as the tank cycles. When the system can process 2 ppm of ammonia to zero within 24 hours with no nitrite, it's ready for its first hardy fish or clean-up crew. Our cycling guide walks through the process, and browse all nitrifying bacteria.
Getting through the "ugly phase"
Brown diatoms, then green hair algae and sometimes cyanobacteria are normal in new dry-rock tanks. Keep up water changes, don't overfeed, run good mechanical filtration and add snails and hermit crabs once the tank has cycled. Coralline algae will gradually spread across the rock; adding a coral or a piece of mature rock from an established tank speeds this up. Most tanks settle within three to six months.
For a complete plan, read How to Set Up a Marine Aquarium and A Beginner Reefkeeper's Shopping List.
