What is Live Rock?

If you are new to saltwater aquariums, "live rock" is one of the first terms you will run into and one of the least obvious. The rock is not alive. What lives is everything on it and inside it.

What makes rock "live"

Most Live Rock starts as the calcium carbonate skeleton left behind by hard corals. Other Live Rock is man made in Fiji, using volcanic sand and fossilized shells pasted together by hand into a unique and interesting shape. It is riddled with pores and tunnels, and that structure is the point — it gives an enormous amount of surface area, inside as well as out, for things to live on.

Put that rock in the ocean and it fills up. Bacteria colonize the pores. Coralline algae spreads across the surface. Sponges, worms, tiny crustaceans and dozens of other organisms move in. Once that has happened, the hobby calls it live rock. The word describes the community on the rock, not the rock itself.

What it does in a reef tank

Three jobs at once. It is the tank's biological filter, with bacteria processing waste in the pores. It is the structure your corals are mounted on and your fish swim through. And it is habitat, housing the small organisms that graze, scavenge and feed the things above them.

Most equipment does one job. Live rock does all three, continuously, without power. Why ocean farmed rock goes into what that biology is worth in practice.

The words you will see

• Live rock — rock carrying an established community of bacteria and marine organisms.
• Dry rock — the same mineral structure with nothing living on it. Cheaper, sterile, and it has to be seeded and cycled before it does anything.
• Ocean farmed, or aquacultured — rock deliberately planted on a licensed ocean site and left to be colonized, then harvested. What we grow.
• Wild harvested — rock taken directly from a natural reef. Illegal or banned in most places, for good reason.
• Cured — rock that has been held somewhere while whatever died in transit finished decaying.

What is not live rock

The term gets stretched, so it is worth being blunt about what does not qualify. Plastic and resin rock is decoration. Dry rock is mineral structure with nothing living on it. Painted rock is dry rock with color added where coralline would be. And rock that is merely wet is not the same as rock that has stayed submerged — wet rock has usually been out of the water long enough for a good deal of what was living on it to die.

None of those are frauds, and several have their uses. They simply are not doing the biological work that the word "live" is describing.

How much your tank needs

The usual guide is about 2 pounds per gallon, so a 40 gallon tank wants somewhere near 80 pounds. Less will work, but you are then asking equipment to do what rock would have done for free.

Worth saying plainly: weight is a rough proxy. Two pieces of the same weight can differ a lot in how porous they are, and porosity is what actually matters. Dense, heavy rock is doing less per pound than light, open rock.

Curing, and why it is not always needed

Curing exists because of how rock is usually shipped. Take living rock out of water, send it damp in a box for a day or two, and a portion of what is living on it dies. That die-off releases ammonia, so the rock is held in a separate container, with water changes, until the decay has finished — often two to six weeks.

Rock that travels submerged does not go through that, because nothing dried out and nothing died. There is no die-off to wait out. This is the single biggest practical difference between suppliers, and it is worth asking any seller how their rock travels before you buy.

Hitchhikers

Anything genuinely alive arrives with passengers. Crabs, shrimp, worms, snails, brittle stars, small corals you did not order. Around 98% of what travels on our rock is beneficial, and most reef keepers end up regarding the hitchhikers as part of what they paid for.

A small number are not welcome in a display tank, which is why it is worth looking your rock over before it goes in, and asking if you are unsure. Nobody can promise a sterile delivery from something that was alive on the seafloor a few days earlier, and a seller who promises one is telling you something about their rock.

Removing the ones you do not want is usually straightforward: a net, a turkey baster, or a trap will handle almost anything you find. Send us a photo if you are not sure what you are looking at — after forty years there is not much we have not seen come up on a rock.

Please don't kill it

Rock that arrives alive can still be killed in the first hour. Three things to avoid:

• Don't cure it — curing is for rock that arrived sick or mishandled. Curing healthy submerged rock just starves the life you paid for.
• Don't dip it — dips are indiscriminate. They kill the beneficial organisms along with whatever you were aiming at.
• Don't leave it in the box — it needs life support the moment it arrives. Unpack it and get it into water straight away, even if you are not ready to aquascape.

Is it a one-time purchase?

Essentially, yes. Rock does not wear out. The community living on it shifts over the years as your tank matures, and most people add a piece here and there as they aquascape, but the rock you buy at the start is still working a decade later. Measured over that span it is among the cheapest things in the tank.