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    Reef-keeping glossary: terms and abbreviations

    Reef keeping is full of abbreviations: AIO, ATO, dKH, PAR, SPS. Here each one is in a sentence or two of plain English, with a link to the page that covers it.

    Last checked . How we check

    A

    AEFWAlso: Acropora-eating flatworm
    A small, coral-coloured flatworm that eats Acropora tissue and lays eggs on the skeleton. Dips kill the adults but not the eggs, so treatment is repeated. Pests and hitchhikers.
    AiptasiaAlso: glass anemone
    A small brown pest anemone that spreads quickly, stings corals and regrows from fragments left behind. Aiptasia profile.
    AlkalinityAlso: KH, carbonate hardness, alk
    A measure of the water’s buffering capacity, mostly carbonate and bicarbonate. Stony corals use it to build skeleton, so it falls as they grow and must be replaced. Alkalinity swings.
    All-in-one (AIO)
    A tank with the filter, heater and return pump hidden in chambers at the back, instead of a separate sump underneath. The usual choice for a first tank.
    Ammonia
    The toxic waste fish and rotting food produce. In a cycled tank, bacteria turn it into nitrite and then nitrate. Any reading above zero in an established tank is a warning.
    ATOAlso: auto top-off
    A sensor and pump that replace evaporated water with fresh RO/DI water, keeping salinity steady. A stuck or empty ATO is one of the commonest causes of tank losses. ATO failure.

    B

    Balling method
    Dosing calcium, alkalinity and a salt mix without sodium chloride as separate solutions, so the water’s ion balance holds over time.
    Bioload
    How much waste the livestock and feeding put into the tank. More fish and more food mean a higher bioload.

    C

    CITES
    The international treaty that controls trade in threatened species. All stony corals are listed, so they need permits to cross borders. Soft corals, zoanthids, mushrooms and anemones are not listed. UK guide: CITES.
    Clean-up crew (CUC)
    Snails, hermit crabs and other invertebrates that graze algae and eat leftovers. Numbers should match the food the tank produces. Invertebrates.
    CopepodsAlso: pods
    Tiny crustaceans that live on rock and glass. Harmless and useful: they are food for mandarins and many small fish.
    Copper treatment
    A standard medicine for marine whitespot and velvet in fish. It kills invertebrates and is hard to remove from rock and sand, so it is used only in a separate treatment tank. Quarantine.
    Coral dip
    A short bath in a treatment solution to knock pests off a new coral before it goes into the tank. No dip reliably kills pest eggs. What each dip does.
    CyanobacteriaAlso: cyano, red slime
    Bacteria that form red, purple or dark green mats on sand and rock, usually where flow is weak or nutrients are out of balance. Cyanobacteria.
    CycleAlso: nitrogen cycle, cycling
    Growing the bacteria that turn ammonia into nitrite and nitrate before livestock go in. The cycle is done when a dose of ammonia clears to zero, through nitrite, within about a day.

    D

    Diatoms
    Single-celled algae that form brown dust on sand and glass in new tanks, fed by silicate. Usually a passing stage. Diatoms.
    DinoflagellatesAlso: dinos
    Single-celled organisms that form brown, snotty strands with bubbles, often in tanks with nitrate and phosphate near zero. Dinoflagellates.
    dKH and meq/L
    The two common units for alkalinity. 1 meq/L equals 2.8 dKH, or 50 ppm as calcium carbonate. Natural seawater is about 6.4 to 6.7 dKH.
    Dosing pumpAlso: doser
    A small pump that adds measured amounts of supplement through the day. Keep its bottles small, so a stuck pump cannot empty a month of supplement at once. Overdose.

    F

    Frag
    A small piece of coral cut from a larger colony and grown on a plug. Most corals in UK shops are sold as frags.

    G

    GFOAlso: granular ferric oxide
    A filter medium that removes phosphate. Easy to overuse in a young tank, taking phosphate to zero.

    I

    IchAlso: marine whitespot, Cryptocaryon
    A common parasite of marine fish that shows as separate white spots like grains of salt. Different from freshwater whitespot. White spots on fish.
    ICP test
    A laboratory analysis of a water sample for dozens of elements at once. Used by the Triton method every two to three months.

    K

    KalkwasserAlso: kalk, limewater
    Calcium hydroxide dissolved in top-off water. It adds calcium and alkalinity and raises pH, so it must be added slowly.

    L

    Live rock and dry rock
    Live rock comes from the sea or an established tank carrying bacteria and life, including pests. Dry rock is clean and pest free but starts with no life on it.
    LPSAlso: large polyp stony coral
    Stony corals with large fleshy polyps, such as hammer, torch and Duncan corals. Usually a step up from soft corals. Coral profiles.

    M

    Magnesium
    A major element in seawater, about 1,280 ppm. Low magnesium makes calcium and alkalinity harder to hold steady.

    N

    NitrateAlso: NO3
    The end product of the cycle. Some is food for corals; too much feeds algae. Most current sources want it measurable rather than zero.
    NitriteAlso: NO2
    The middle step of the cycle, between ammonia and nitrate. It should read zero once the tank is cycled.

    P

    Palytoxin
    A strong toxin carried by some zoanthids, especially Palythoa. Never scrub, cut, scald or boil them; wear gloves and eye protection. Palythoa safety.
    PARAlso: photosynthetically active radiation
    A measure of how much usable light reaches a coral, read with a PAR meter. Soft corals need less than SPS.
    PhosphateAlso: PO4
    A nutrient that, with nitrate, feeds algae and corals. Common targets run from about 0.02 to 0.1 ppm.
    PrecipitationAlso: snowstorm
    Calcium carbonate coming out of the water as white cloud or crust, usually after dosing too much alkalinity or kalkwasser at once. Cloudy water.
    Protein skimmer
    A device that removes waste by foaming it into a collection cup. Usual on most tanks, optional on some small soft-coral tanks.

    Q

    Quarantine (QT)
    Keeping new fish or corals in a separate tank for weeks so disease and pests show up before they reach the display. Quarantine.

    R

    RCDAlso: residual current device
    A safety switch that cuts the power if current leaks to earth, for example through water. Standard UK advice for every aquarium circuit. Leak or flood.
    Refractometer
    A small optical tool for measuring salinity. Calibrate it in 35 ppt seawater fluid, not RO water.
    Refugium
    A lit chamber in the sump that grows macroalgae to take up nutrients and shelter small life such as copepods.
    RO/DIAlso: reverse osmosis and deionisation
    A filter unit that turns tap water into pure water for mixing salt and topping off. Its output should read 0 TDS.

    S

    Salinity, ppt and SG
    How salty the water is. Most sources aim for 35 ppt (parts per thousand), about 1.026 specific gravity (SG) at 25 C.
    SPSAlso: small polyp stony coral
    Stony corals with tiny polyps, such as Acropora and Montipora. They need high light, strong flow and very steady water. Acropora profile.
    Sump
    A second tank, usually in the cabinet, that holds the skimmer, heaters and return pump and adds water volume.

    T

    Tank transfer method (TTM)
    Moving a fish into a clean, dry-started tank every 72 hours so ich parasites are left behind. Works for ich only, not velvet. Quarantine options.
    TDSAlso: total dissolved solids
    A meter reading of dissolved material in water. RO/DI output should read 0; a rising number means the resin needs changing.

    U

    Ultra-low nutrient system (ULNS)
    A style of reef keeping that holds nitrate and phosphate very low for colour. Less forgiving, and linked to pale corals and dinoflagellates.

    V

    VelvetAlso: Amyloodinium
    A fast-killing fish parasite that looks like fine dust, often with rapid breathing. Needs action the same day. White spots on fish.

    W

    Water change
    Replacing part of the tank water with fresh salt water to dilute waste and restore elements. About 10% a week is common.

    Z

    ZoanthidsAlso: zoas
    Colonial soft corals with small, colourful polyps. Easy to keep, but treat every one as possibly carrying palytoxin. Zoanthid profile.

    Sources

    1. Red Sea, Foundation testing and supplementing manual (v21a)Published 2021. Read 21 October 2026, read in full.
    2. Wikipedia, Carbonate hardnessRead 21 October 2026, read in full.
    3. GOV.UK, CITES imports and exportsPublished 23 June 2026. Read 21 October 2026, read in full.
    4. Cefas marine science blog, Safeguarding the global coral tradePublished 15 January 2024. Read 21 October 2026, read in full.
    5. Bulk Reef Supply, Auto top-off and evaporation FAQPublished 6 February 2024. Read 21 October 2026, read in full.
    6. Bulk Reef Supply, Dr Tim’s fishless cycling recipePublished 2019. Read 21 October 2026, read in full.
    7. Bulk Reef Supply, Best coral dips of 2019Published 2019. Read 21 October 2026, read in full.
    8. US CDC, MMWR: suspected palytoxin inhalation exposures, Alaska 2012 to 2014Published August 2015. Read 21 October 2026, read in full.
    9. Charterhouse Aquatics, What should you test for in a reef aquariumPublished 2 July 2026. Read 21 October 2026, read in full.