Cycling a Reef Tank with Dry Rock

How dry base rock differs from live rock, why it can release phosphate, how an ammonia source and seeding start nitrification, and why dry-rock tanks mature more slowly.

Dry rock versus live rock

Live rock is ocean rock placed in a marine aquarium. The rock itself is not alive: it is the aragonite skeleton of long-dead corals and other calcareous organisms, and it counts as live because bacteria, algae, sponges and invertebrates colonise its surface and pores. Base rock, usually called dry rock, is aragonite rock with nothing growing on or in it. It may be natural rock, often mined from ancient reefs inland, or artificial rock cast from crushed aragonite, sand and cement. Dry rock costs less than live rock and is gradually colonised once it sits in an aquarium.

The trade-off runs in both directions. Dry rock brings no pest hitchhikers such as aiptasia anemones, mantis shrimp or parasitic flatworms, and because nothing on it can die, the aquascape can be built and rebuilt slowly. It also brings no microfauna and no ready-made nitrifying community, so the biological filter has to be grown from scratch. A collection ban is another factor in the background: CITES banned live rock collection from Tonga, the Marshall Islands and the Cook Islands in 2008 because of over-collection; verify current status.

Phosphate and organic residue in dry rock

Natural dry rock is a dried coral skeleton with a deep network of pores. Hobby editorial sources note that these pores can hold dried organic matter, such as dead sponges, small fauna and plant material, and that it may leach phosphate into the water as it breaks down. Phosphate released this way can feed the algae blooms typical of new tanks. In the aquarium study of Bik and colleagues, measured phosphorus was low at setup and rose after live rock and established sediment were added, although phosphorus readings fluctuated through the whole experiment.

The practical consequence is to measure rather than assume. Soaking and circulating the rock in saltwater before use, and testing that water for phosphate, shows whether a given batch releases it. Where a batch does, the same hobby source mentions phosphate-adsorbing media as a temporary measure. No scientific source reviewed here gives a figure for how much phosphate dry rock releases or for how long, and none that compares types of rock.

Starting nitrification: the ammonia source

Nitrification converts ammonia to nitrite and then to nitrate. In a dry-rock tank the microbes that do this must arrive and multiply before animals are added, and they need a supply of ammonia to grow on. Public aquariums use the same approaches hobbyists do: seeding with live rock, filter media or substrate from established systems, adding fish food, dosing pure ammonia, or adding bottled nitrifying cultures. When the Shedd Aquarium restarted a disinfected 5,000-gallon saltwater exhibit on new aragonite sand, staff started ammonia oxidation with a squid and fish slurry and then dosed ammonium chloride in rising amounts for about a month before the first animals went in.

The ammonia supply is temporary and measured. In the same paper, ammonia is described as toxic to marine animals from about 0.2 ppm. Regular ammonia and nitrite tests show when the developing filter keeps pace with the dose; the Shedd authors stress giving the system enough time and ammonia input before animals are introduced. How pH and temperature shift the balance between toxic ammonia and ammonium is covered in the ammonia-ammonium-equilibrium-guide; the general sequence is in the nitrogen-cycle-guide.

Which microbes do the work

The Shedd team expected ammonia-oxidising bacteria to dominate the start-up. Instead, ammonia-oxidising archaea of the genus Nitrosopumilus were the main ammonia oxidisers and peaked while ammonium was being added. Complete ammonia oxidisers (comammox Nitrospira), common in freshwater filters, have not been found in saltwater aquaria. The authors conclude that a new saltwater system needs enough time and enough ammonia to build a strong archaeal population before animals arrive. The microbes themselves are described in the aquarium-microbiome-and-biofilm-guide and the ras-nitrification-microbiology-guide.

Bacterial inoculation and seeding

The strongest evidence for speeding up a sterile start comes from transferring material from a working system. In the study by Bik and colleagues, two new saltwater aquaria built from commercial sand showed ammonia, nitrite and nitrate rising for the first 12 days. When cured live rock and sediment from an established aquarium were added on day 12, ammonia and nitrite dropped immediately, and the sediment community shifted within hours toward a typical saltwater assemblage that included nitrogen-cycling taxa. The authors conclude that live rock improves nutrient cycling in a new tank. A few pieces of live rock or a portion of live sand added to dry rock serve the same purpose and also bring microfauna.

Bottled nitrifying cultures are a widely used inoculum, but the Shedd paper states that little is known about how well they or other seeding practices work. No peer-reviewed source reviewed here shows that a bottled product shortens cycling on dry rock.

Timeline compared with live rock

No fixed duration applies. In the Shedd system, ammonium dosing started on day 10 and live foods were added on day 37 and fish on day 41. A hobby editorial account of a dry-rock reef cycled with ammonium chloride and bacterial additives reported that it took noticeably longer than the few weeks usual for live-rock systems. Water tests, not the calendar, decide when the filter is ready.

Finishing the nitrogen cycle does not mean the tank is mature. The same account describes algae blooms on dry rock as more persistent and widespread than on live rock, which the author attributes to the many empty niches on bare rock; the blooms eventually cleared with routine maintenance. Hobby sources list diatoms, cyanobacteria and dinoflagellates as the usual setbacks in new tanks. That sequence is covered in the reef-tank-ugly-phase-and-maturation-guide, and stony rock with established life in the live-rock-guide.