🐠 Reef glossary

Saltwater & reef terms, in plain language

Every hobby has its jargon, and reefkeeping is thick with it. Here are the words you'll meet — in plain language, no gatekeeping, no assumed knowledge. 54 terms, each linked to the guide or tool that goes deeper. Because reefkeeping is water-keeping, we start with the chemistry.

Updated September 2026

💧 Water & chemistry

AlkalinitydKH · KH · carbonate hardnessAlkalinity is the water's buffering capacity — the carbonate and bicarbonate that resist pH swings and that stony corals use to build their skeletons — targeted at roughly 8–11 dKH on a reef.AmmoniaNH3Ammonia is the toxic first waste product fish and decay produce; in a cycled tank the beneficial bacteria keep it at zero, and any reading above zero is an emergency.CalciumCaCalcium is the dissolved mineral stony corals and coralline algae use to build calcium-carbonate skeletons, kept at about 400–450 ppm on a reef.DinoflagellatesdinosDinoflagellates ("dinos") are a slimy, stringy, often brown pest microorganism — frequently toxic to snails and corals — that blooms when nutrients are stripped too low.Kalkwasserlimewater · kalkKalkwasser ("lime water") is a saturated calcium-hydroxide solution dripped into the tank to replenish calcium and alkalinity at once while helping hold pH up.MagnesiumMgMagnesium is the mineral that keeps calcium and alkalinity in solution together, held at roughly 1300–1400 ppm — get it right and the other two become far easier to stabilise.NitrateNO3Nitrate is the relatively harmless end product of the nitrogen cycle — but on a reef you want it low, not zero, around 2–15 ppm, because corals need some nutrients.NitriteNO2Nitrite is the toxic intermediate the nitrogen cycle produces between ammonia and nitrate; like ammonia, it must read zero in a cycled tank.PARPhotosynthetically Active RadiationPAR (Photosynthetically Active Radiation) measures the amount of usable light reaching your corals — the honest way to judge coral lighting, since watts and brightness don't tell you what a coral actually gets.pHpH measures how acidic or basic the water is; a reef runs slightly basic, around 8.1–8.4, and stability matters more than chasing an exact figure.PhosphatePO4Phosphate is a nutrient from food and waste that feeds algae when high but is needed in small amounts by corals — the reef target is low-but-not-zero, roughly 0.03–0.10 ppm.RODI waterRO/DI · reverse osmosisRODI is reverse-osmosis / deionised water — tap water purified of the nitrate, phosphate, silicates and metals that feed algae and harm inverts — and it's what you mix your salt with.SalinitypptSalinity is the concentration of dissolved salt in the water, measured in parts per thousand (ppt); a reef tank targets about 35 ppt — the same as natural seawater.Specific gravitySGSpecific gravity (SG) is the density of your saltwater compared with pure water — a temperature-dependent proxy for salinity, with a reef target around 1.026 at 77°F (25°C).Two-part dosing2-part · B-ionicTwo-part is a matched pair of liquid supplements — one calcium, one alkalinity — dosed together to keep both in balance as corals consume them.

🔄 Cycling & biology

Beneficial bacterianitrifying bacteriaBeneficial bacteria are the nitrifying microbes — living mostly on your rock and in the sand and filter — that convert toxic ammonia and nitrite into safer nitrate.Cyclingfishless cycleCycling is the weeks-long process of growing the beneficial bacteria that run the nitrogen cycle, done before you add livestock — the tank is cycled when it turns a dose of ammonia to zero ammonia and zero nitrite within a day.Diatomsbrown algae · brown dustDiatoms are the brown, dusty film of algae that coats rock, sand and glass in a new tank — a harmless, normal phase of a maturing reef.Live rockdry rock · reef rockLive rock is porous calcium-carbonate rock that houses the beneficial bacteria and micro-life running your reef's biological filtration — the foundation of the aquascape.New tank syndromethe ugly stageNew tank syndrome — the "ugly stage" — is the normal run of diatom and algae blooms a young reef goes through as it matures and finds its biological balance.Nitrogen cyclethe cycle · biological cycleThe nitrogen cycle is the process where beneficial bacteria convert toxic fish waste (ammonia) into nitrite, then into far less harmful nitrate — the single most important thing to understand in the hobby.

⚙️ Equipment & gear

Auto top-offATOAn auto top-off (ATO) automatically replaces the freshwater that evaporates, keeping your water level — and therefore your salinity — stable day to day.Fragfragging · fragmentA frag is a small cutting of a coral, mounted on a plug or disc, used to propagate it or trade it — how most corals are bought and shared in the hobby.Powerheadwavemaker · flow pumpA powerhead is a submersible pump that creates water flow inside the display — keeping detritus in suspension, feeding corals and gas-exchanging the surface.Protein skimmerskimmer · foam fractionatorA protein skimmer uses a column of fine bubbles to pull dissolved organic waste out of the water as foam, before it breaks down into nitrate and phosphate.QuarantineQT · quarantine tankQuarantine (QT) is isolating new fish in a separate, cycled tank for a few weeks to observe and treat them before they join the display — the best defence against introducing disease.RefugiumfugeA refugium is a protected section of the sump where macroalgae and copepods grow — exporting nutrients as the algae grows and seeding the tank with live food.Return pumpThe return pump is the pump that sends water from the sump back up to the display, driving the whole sump's circulation.SumpA sump is a second tank plumbed below the display that adds water volume and hides equipment — skimmer, heater, return pump and often a refugium — out of sight.

🐠 Corals

Allelopathychemical warfareAllelopathy is the chemical warfare corals wage on each other — releasing compounds that stunt or harm rivals — a battle softies and leathers wage even without touching.Coral bleachingBleaching is when a stressed coral expels its zooxanthellae and turns pale or bone-white — most often from too much light too fast, a temperature swing, or unstable water.Coral dipdippingA coral dip is a short bath in a treatment solution that kills or dislodges pests and parasites on a new coral before it goes into your display.Coralline algaeCoralline algae is the hard, encrusting pink and purple algae that spreads over rock and glass in a mature reef — a welcome sign of stable, healthy water.LPS corallarge-polyp stonyLPS (large-polyp stony) corals — hammer, torch, frogspawn, acan, duncan — are fleshy, feedable corals that build a stony skeleton and need stable alkalinity, calcium and magnesium.PolypA polyp is the individual coral animal — a small sac with a mouth ringed by tentacles; what we call "a coral" is usually a colony of many connected polyps.RTN / STNrapid tissue necrosis · slow tissue necrosisRTN (rapid) and STN (slow) tissue necrosis describe coral tissue dying and sloughing off the skeleton — an SPS emergency that can strip a colony to bare white bone in hours to days.Soft coralsoftiesSoft corals are corals without a hard stony skeleton — zoanthids, mushrooms, leathers, GSP, xenia — and the forgiving, low-demand corals most keepers start with.SPS coralsmall-polyp stonySPS (small-polyp stony) corals — acropora, montipora, birdsnest — are the high-light, high-flow branching corals that demand pristine, rock-stable water; the advanced tier of the hobby.Sweeper tentaclesSweeper tentacles are the long, extended stinging tentacles some corals — Euphyllia like hammer, torch and frogspawn especially — reach out at night to attack and clear space around neighbours.Zooxanthellaezoox · symbiotic algaeZooxanthellae are the microscopic symbiotic algae living inside coral tissue that photosynthesise, feed the coral most of its energy, and give it much of its colour.

🐟 Fish

Captive-bredtank-bred · aquaculturedCaptive-bred (or aquacultured) livestock is raised in captivity rather than collected from the wild — usually hardier, better adapted to tank life, and with no impact on wild reefs.Conspecificown kindConspecific means "of the same species" — the word behind own-kind aggression rules like "one tang per tank" or "keep chromis in a group of six."FOWLRfish-only with live rockFOWLR (Fish-Only-With-Live-Rock) is a marine tank kept with fish and live rock but no corals — the simplest, most forgiving way into saltwater.Reef-safe"Reef-safe" describes an animal that won't eat or destroy corals and clean-up inverts — but it's a spectrum, and many fish are reef-safe "with caution" (they may nip clams or LPS, or eat shrimp).

🦐 Inverts & cleanup

Cleanup crewCUCThe cleanup crew (CUC) is the team of snails, hermit crabs and other inverts that graze algae, turn the sand and scavenge leftover food — the reef's janitorial staff.CopperCopper is a metal that is lethal to all invertebrates and corals even in trace amounts — present in many fish medications — so it must never enter a reef tank.MoultingmoltingMoulting is when a shrimp, crab or other invertebrate sheds its old exoskeleton to grow a larger one — the discarded empty shell is a healthy sign, not a death.

🏥 Health & pests

Aiptasiaglass anemone · pest anemoneAiptasia is a fast-spreading pest anemone that stings corals and multiplies rapidly — and scraping it off only spreads it, so it's removed by predators or targeted treatment.Brown jellybrown jelly diseaseBrown jelly is a fast-moving brown, gelatinous infection that can consume a fleshy LPS coral — torch and other Euphyllia especially — in a matter of days.Marine ichwhite spot · CryptocaryonMarine ich (Cryptocaryon irritans) is a common parasite that shows as tiny white salt-grain spots on a fish, usually triggered by stress or a temperature or parameter swing.Marine velvetvelvet · AmyloodiniumMarine velvet (Amyloodinium) is a faster, deadlier parasite than ich — a fine dusty or velvety sheen on the fish with rapid breathing — that can wipe out a tank in days.

📖 General

Acclimationdrip acclimationAcclimation is gradually adjusting new livestock to your tank's temperature and salinity before releasing it — slowly, since sudden change is a major cause of stress and loss.Salt mixsynthetic sea salt · reef saltSalt mix is the synthetic sea-salt blend you dissolve in RODI water to make saltwater — reef blends carry extra calcium, alkalinity and magnesium for coral tanks.Top-offtopping offTopping off is replacing water lost to evaporation with fresh RODI water — because only water evaporates, the salt stays behind and the tank would otherwise creep saltier.