Reef Terms, Explained

The reef hobby runs on shorthand. If you are new, or buying a gift for someone who is, here is what the words on our product pages actually mean. No sales pitch, just definitions.

Frag

A small cutting taken from a larger coral. Corals are colonies of tiny identical polyps on a shared skeleton, and a piece broken off can grow into a whole new colony. It happens naturally when storms break up a reef.

Fragging

Deliberately cutting a coral into pieces to grow more of it. Reef keepers do this to control growth, to trade and sell, to save a coral that is dying at the base, and because a frag costs a fraction of a full colony. It is most of the reason the hobby has a social scene: frag swaps, reef clubs and shows exist because of it.

Frag plug

The small ceramic disc with a stem that a coral cutting is usually glued to. It is the standard unit corals are sold on. Plugs are cheap and easy to handle, but they look obviously artificial in a display tank, and coral often grows over the edge so you end up cutting the plug away later.

Frag rack

A perforated acrylic tray that holds a row of frag plugs. Efficient, and popular in dedicated frag tanks and sumps, but it stays visibly like equipment.

Frag rock

What we make. Real reef rock shaped to hold coral cuttings, so it does a rack's job but blends into your aquascape instead of sitting there looking like hardware.

Coralline

A hard, crusty purple-pink algae that slowly grows over rock and glass in a mature saltwater tank. Reef keepers treat it as a good sign, because it means the water chemistry and stability are where corals want them. On dry rock it typically takes many months to a year to appear, and it has to be seeded from live rock, a frag or a bottled product. It will not appear on its own.

Worth being straight about: when you pick "Coralline" as a color option on our products, you are choosing rock finished with a reef-safe organic colorant. You are not buying living algae, and no dry rock product ships with live coralline on it. The colored option exists so a brand new tank does not have to look bone white for a year. Real coralline grows over it as your tank matures.

Natural

The rock's own color, a white to tan tone. It is what dry reef rock actually looks like, and it will color up on its own over time as your tank matures.

Dry rock

Reef rock that arrives with nothing living on it. It carries no pests, no nuisance algae and no hitchhikers, so you control exactly what enters your tank. The trade-off is that it brings no bacteria either, so the tank has to be cycled. Everything we build is handcrafted from dry MarcoRocks.

Live rock

Rock already carrying bacteria and organisms, from the ocean or an established tank. It seeds a tank faster, but it can bring in pests such as aiptasia, mantis shrimp or flatworms that are hard to remove later.

Cycling

Establishing the bacteria that process fish waste, turning ammonia into nitrite and then nitrate. It takes weeks and is the first thing a new tank goes through. Adding a modest piece of clean dry rock to an established tank does not restart this.

Curing

Cleaning die-off from rock, so that decaying organisms do not spike your ammonia. This applies to wild live rock. Dry rock has nothing living on it to die, so it does not need curing in that sense.

Reef safe

Will not harm corals, fish or invertebrates.

Inert

Does not meaningfully change your water chemistry. It will not leach phosphate, metals or anything else that feeds algae or stresses coral.

Aquascape (or rockscape)

The arrangement of rock inside the tank. It decides where corals can sit, how water moves, and whether the finished tank reads as a reef or a pile.

Negative space

The open water and bare sand you deliberately leave empty. It is not wasted room. Packing a tank wall to wall blocks flow, and where flow stops, waste settles and rots. Open space also makes a tank look bigger and gives fish somewhere to swim.

SPS and LPS

Small Polyp Stony and Large Polyp Stony corals. As a rough rule SPS want the strongest light and flow, so they go high in the scape, while LPS and soft corals prefer it gentler and sit lower or in shade. Getting height into your rockwork is how one tank serves both.

PAR

A measure of how much usable light actually reaches a coral. It drops off quickly with depth, which is why height in a scape matters and why people move new corals up gradually rather than putting them straight under the brightest spot.

Encrusting

A coral growing its tissue outward over whatever it is attached to. It is why a frag mounted on natural rock eventually looks like it grew there.

Sump

A second tank hidden below the display that holds equipment and extra water.

Refugium

A protected compartment, usually in the sump, where copepods and macroalgae can grow without being eaten. Rubble and a light are the usual ingredients.

Rubble

Small broken chunks of reef rock. Used for filtration surface area, as copepod habitat in a refugium, as bases for coral cuttings, and for hiding joints in a scape.

Copepods

Tiny crustaceans that feed fish and corals. They breed in the nooks of porous rock, which is why rubble goes in a refugium.

Byssal gland

The organ a clam uses to spin threads and anchor itself to hard rock. It is the reason rock-dwelling clams such as maxima and crocea need a solid seat rather than bare sand, and why a clam that gets knocked over after anchoring is in real trouble.

FOWLR

Fish Only With Live Rock. A saltwater tank with fish and rock but no corals.

Still not sure what you need?

Tell us your tank size and what you are keeping and we will point you at the right piece, or build you something specific. Start a custom build or get in touch.