The reef-keeper's glossary
Every saltwater aquarium term you'll see across our guides, calculators, and reviews — defined for reefers, by reefers.
A
- Acclimation
- The process of slowly adjusting new fish or corals to your tank's temperature, salinity, and pH before introducing them. Drip acclimation over 30–60 minutes is standard.
- AIO (All-in-One)
- A reef tank with built-in filtration chambers in the back instead of an external sump. Examples: Fluval Evo, Innovative Marine NUVO, Waterbox AIO.
- Alkalinity (Alk / dKH)
- A measure of the water's buffering capacity, expressed in degrees of carbonate hardness (dKH). Reef target: 7.5–9.5 dKH. Stability matters more than the exact number.
- Anemone (BTA)
- Bubble-Tip Anemone — a popular host for clownfish. Mobile invertebrate that can sting nearby corals; assume it will roam.
- Aquascape
- The intentional arrangement of live rock and sand in a reef tank to create structure, flow channels, and coral placement zones.
- ATO (Auto Top-Off)
- A device that automatically refills evaporated water with fresh RO/DI to keep salinity stable. Brands: Tunze Osmolator, XP Aqua Duetto.
B
- Bioload
- The total biological waste produced by livestock in a tank. Higher bioload requires more filtration and water changes.
C
- Calcium Reactor (CaRx)
- An automated dosing device that dissolves calcium media with CO₂ to maintain alkalinity, calcium, and magnesium for SPS-heavy reefs.
- Cycling
- The 4–6 week process of establishing nitrifying bacteria in a new reef tank. Ammonia → nitrite → nitrate. No livestock until both ammonia and nitrite read zero.
- Cleanup Crew (CUC)
- Snails, hermit crabs, and shrimp that eat algae, detritus, and uneaten food. Standard ratio: ~1 small snail per gallon for nano reefs.
D
- Dosing
- Adding alkalinity, calcium, magnesium, or trace elements to maintain stable parameters as corals consume them.
F
- Frag
- A small cutting of a coral colony, usually mounted on a ceramic plug. Frags grow into colonies over months to years.
G
- GFO
- Granular Ferric Oxide — a phosphate-removing media used in reactors to keep PO₄ low (target: 0.02–0.10 ppm).
- GSP
- Green Star Polyps — a beginner-friendly soft coral. Spreads aggressively; isolate on its own rock or it will overtake the tank.
K
- Kalkwasser
- Calcium hydroxide solution dosed to replenish alkalinity and calcium simultaneously. Drips into the sump or via ATO at night.
L
- LPS
- Large Polyp Stony coral — fleshy, large polyps. Examples: hammer, torch, frogspawn, acan, chalice. Moderate-light, low-flow placement.
- Live Rock
- Aquacultured or wild-harvested calcium-carbonate rock with bacteria and microfauna already established. Speeds up cycling but may carry pests.
N
- Nori
- Dried seaweed sheets used to feed herbivorous reef fish like tangs. Clip to the glass with a magnetic veggie clip.
P
- PAR
- Photosynthetically Active Radiation — the spectrum of light corals use for photosynthesis. SPS: 250–400 PAR; LPS: 100–200 PAR; soft: 60–120 PAR.
Q
- Quarantine (QT)
- A separate hospital tank used to observe and treat new fish for 14–30 days before adding to the display. The single biggest preventer of full-tank wipe-outs.
R
- Refractometer
- A precision instrument that measures salinity by light refraction. More accurate than swing-arm hydrometers. Reef target: 1.025–1.026 specific gravity.
- Refugium
- A separate compartment (usually in the sump) growing macroalgae and copepods to remove nutrients and feed reef-fish.
- RO/DI
- Reverse Osmosis / Deionization — a multi-stage water filter that produces zero-TDS water for top-off and salt mixing. Essential for reef tanks.
- RTN
- Rapid Tissue Necrosis — sudden coral tissue loss in SPS, usually from alkalinity swings, salinity instability, or pest predation.
S
- Skimmer
- Protein skimmer — removes dissolved organic compounds before they break down into nitrate. Sized at ~1.5× tank volume turnover per hour.
- SPS
- Small Polyp Stony coral — branching, high-light demand. Examples: Acropora, Montipora, Stylophora, Seriatopora. Requires stable parameters and high flow.
- STN
- Slow Tissue Necrosis — gradual coral tissue loss from base upward. Usually nutrient or chemistry related; addressable if caught early.
- Sump
- A secondary tank below the display that houses skimmer, return pump, heater, and reactors. Increases water volume and hides equipment.
T
- TTM
- Tank Transfer Method — a copper-free quarantine protocol that uses 4 transfers across 12 days to break the ich life cycle.
U
- ULNS
- Ultra-Low Nutrient System — an SPS-keeping approach that maintains very low nitrate (1–5 ppm) and phosphate (0.02–0.06 ppm).
W
- Wavemaker
- A propeller pump (e.g., EcoTech MP10/MP40, Maxspect Gyre) that generates flow patterns coral need. Reef target turnover: 20–80× tank volume per hour.
Z
- Zoanthid (Zoa)
- A genus of beginner-friendly soft corals available in thousands of color morphs. Spreads via stolons. Wear gloves — palytoxin can be present in some species.
Reef Library
Keep learning the right way
The reef-keeping classics that taught most of the editorial team — kept on the shelf for daily reference.