The Reefkeeper's Glossary: Frag, ULNS, Alk, and 80 More Terms

Reefkeeping has a vocabulary problem. Open any forum thread and you will hit sentences like "my alk swung after I fragged the monti, and now I think dinos are outcompeting the GHA in my ULNS." Every hobby builds jargon, but reefing borrows from chemistry, marine biology, plumbing, and twenty years of forum shorthand all at once. This glossary defines the terms you will actually meet — in care guides, in fish store conversations, and in the threads you will eventually write yourself. Skim it now, bookmark it, and come back whenever a guide assumes a word you do not know.

Water chemistry

Alkalinity (alk) — The water's capacity to buffer acid, and the raw material stony corals use to build skeleton. The most important number in reefkeeping. Measured in dKH or meq/L.

dKH — Degrees of carbonate hardness, the common unit for alkalinity. Most reefs run 7–11 dKH.

Calcium (Ca) — The other skeletal building block; typically kept at 400–450 ppm.

Magnesium (Mg) — Keeps calcium and alkalinity dissolved and in balance; typically 1250–1350 ppm.

The big three — Alkalinity, calcium, magnesium, managed together. Our reef parameters chart lists targets for all of them, and the Reef Chemistry Handbook explains how they interact.

Nitrate (NO3) — The relatively benign end product of the nitrogen cycle; a nutrient to be managed, not eliminated. Most reefs target 2–10 ppm.

Phosphate (PO4) — The other major nutrient; high levels fuel algae and can slow coral calcification. Typical target 0.03–0.1 ppm.

Salinity — Salt concentration, ideally 35 ppt (parts per thousand), measured with a refractometer rather than a cheap swing-arm hydrometer.

Specific gravity (SG) — Density-based expression of salinity; 1.026 SG corresponds to 35 ppt.

pH — Acidity/basicity of the water; reefs sit around 7.8–8.4, and daily swings are normal.

ORP — Oxidation-reduction potential, a rough proxy for water cleanliness. Widely logged, rarely worth chasing.

Trace elements — Minor elements (iodine, strontium, and dozens more) consumed in tiny amounts; replenished by water changes or targeted dosing.

ICP test — Mail-in laboratory analysis that measures dozens of elements hobby kits cannot.

Two-part — A paired alkalinity and calcium dosing system, added in matched amounts.

Kalkwasser (kalk) — Calcium hydroxide solution dosed via top-off water; supplements alk and calcium while raising pH.

Carbon dosing — Adding a carbon source (vinegar, vodka, biopellets) to feed bacteria that consume nitrate and phosphate.

ULNS — Ultra-low nutrient system, a deliberately nutrient-starved approach once fashionable for SPS color. Demanding, and no longer the default advice.

The cycle and tank biology

The cycle (nitrogen cycle) — The bacterial process that converts toxic ammonia to nitrite to nitrate. Every new tank must complete it before livestock — see how long a saltwater tank takes to cycle.

Ammonia (NH3) — Highly toxic waste product of fish and decay; should read zero in an established tank.

Nitrite (NO2) — The intermediate cycle product; also toxic, also zero in a running reef.

Live rock / dry rock — Reef rock with, or without, established bacteria and life. Dry rock starts sterile and cycles slower but pest-free.

Cured — Rock whose die-off has finished decomposing, safe to add without an ammonia spike.

Diatoms — The brown dusting of algae nearly every new tank grows; harmless and temporary.

The uglies — Forum shorthand for the algae phases (diatoms, film, hair) every young tank cycles through in its first months.

Copepods / amphipods (pods) — Tiny crustaceans that colonize rock and sand; cleanup labor and live food.

Phytoplankton (phyto) — Microalgae dosed as food for pods and filter feeders.

Refugium (fuge) — A protected sump section growing macroalgae to export nutrients and harbor pods.

Chaeto — Chaetomorpha, the wiry green macroalgae most refugiums grow.

Deep sand bed / bare bottom — Competing substrate philosophies: a deep bed for denitrification, or no sand for easy detritus removal.

Old tank syndrome — Slow decline in mature tanks from accumulated detritus and depleted buffering, usually after years of shortcuts.

Equipment

Sump — An auxiliary tank below the display holding equipment and extra water volume.

AIO (all-in-one) — A tank with filtration chambers built into the back wall instead of a sump; the standard nano format.

Overflow — The drain assembly carrying display water down to the sump.

Return pump — The pump sending sump water back up to the display; its rate is the system's turnover.

ATO (auto top-off) — Device that replaces evaporated fresh water automatically, holding salinity stable.

RO/DI — Reverse osmosis + deionization water purification; the standard source water. Quality is verified by TDS (total dissolved solids), which should read zero.

Protein skimmer — Filtration that removes dissolved organics as foam. The collected sludge is skimmate; running wet or dry describes pulling more or less liquid with it.

Media reactor — A chamber that fluidizes filtration media in flow, most often GFO (granular ferric oxide, a phosphate binder) or GAC (activated carbon).

Doser — A programmable peristaltic pump delivering measured amounts of two-part or supplements.

Powerhead / wavemaker — Internal pumps creating flow inside the display.

Heater controller — External thermostat that cuts a heater's power if its internal switch fails.

PAR — Photosynthetically active radiation; the working measure of usable light intensity for corals. See PAR levels for corals.

T5 / LED — The two dominant reef lighting technologies; long-running debate, both proven.

QT (quarantine tank) — A bare holding tank for isolating and observing new livestock before it touches the display.

Corals

SPS / LPS / softies — The hobby's rough coral triad: small-polyp stony corals (acropora, montipora — demanding), large-polyp stony corals (hammers, torches — moderate), and soft corals (leathers, zoas — forgiving). All the practical care differences are covered in Coral Care and Propagation.

Zoas — Zoanthids, colonial button polyps prized for color patterns; named morphs trade like collectibles.

Frag — A deliberate coral fragment mounted for growth or trade; also the verb. A coral fragging kit covers the cutters, glue, and plugs involved.

Frag plug — The small ceramic disc a frag is glued to.

Colony / mother colony — A grown coral; the parent from which frags are cut.

WYSIWYG — What you see is what you get: buying the exact photographed specimen, not one like it.

Aquacultured / maricultured — Grown in captivity, or farmed in the ocean; both preferable to wild-collected.

Polyp extension (PE) — How far a coral extends its polyps; the everyday visual indicator of coral mood.

Sweeper tentacles — Elongated stinging tentacles some corals deploy at night to attack neighbors.

Allelopathy — Chemical warfare between corals, waged through the water rather than by touch.

Coral dip — A pest-killing bath given to new corals before they enter the tank.

Browning out — A colorful coral turning brown, usually from excess nutrients or insufficient light.

Bleaching — A stressed coral expelling its symbiotic algae and turning white; serious but often recoverable.

Zooxanthellae — Those symbiotic algae, which feed the coral through photosynthesis.

RTN / STN — Rapid or slow tissue necrosis: coral tissue peeling from the skeleton, fast or slowly. The alarm bells of stony coral keeping.

Fish and livestock

CUC (cleanup crew) — The snails, hermits, and other invertebrates employed to eat algae and scavenge.

Reef safe — Livestock that will not eat corals or invertebrates. Always verify; the label is applied optimistically.

Ich / velvet — The two common parasitic fish diseases; the reason quarantine exists.

Drip acclimation — Slowly equalizing a new animal to tank water via airline drip.

Hosting — A clownfish adopting an anemone (or a stand-in coral) as home territory.

Nem — Forum shorthand for anemone.

Pests and problems

Aiptasia / majano — Weedy pest anemones that sting corals and multiply relentlessly.

Cyano — Cyanobacteria, the red slime film of nutrient-imbalanced tanks.

Dinos — Dinoflagellates, a snotty brown outbreak that often follows over-cleaning a tank toward zero nutrients.

GHA — Green hair algae.

Bubble algae — Green pearl-like vesicles; popping them in-tank spreads spores.

Bryopsis — A feathery hair algae notorious for resisting normal cleanup crews.

AEFW — Acropora-eating flatworms, a dreaded SPS pest.

Vermetid snails — Tube-dwelling snails whose mucus nets irritate corals.

Hitchhiker — Any organism that arrives uninvited on rock or frags; some welcome, some not.

Hobby shorthand

LFS — Local fish store. DT — Display tank. FTS — Full tank shot. SPS'd — (Joking) having caught the expensive stony coral addiction. Frag swap — A meetup where hobbyists trade frags. Crash — The sudden loss of most of a tank's livestock; the thing every practice in this glossary exists to prevent.

If you are brand new and this list feels like a foreign language syllabus, start with The First Tank — it introduces these ideas in order, as you need them, rather than alphabetically. And when you meet a term in the wild that is not here, the odds are good it is a brand name, a coral morph, or a joke about tangs.

FAQ

Do I need to memorize all of this?

No. You will absorb the chemistry terms in your first month of testing with a reef master test kit, the coral terms as you stock, and the pest terms — with luck — never. This page works better as a reference than a reading assignment.

What is the difference between a frag and a colony?

Size and history. A frag is a cut fragment, usually an inch or less on a plug; a colony is a grown, established coral. Frags cost less and adapt well, which is why beginners are steered toward them — see how to frag zoanthids for how frags are actually made.

Why does everyone abbreviate everything?

Twenty years of forum culture. Alkalinity is typed thousands of times a day across reef forums; it was always going to become alk.