Reef glossary
Plain-English definitions of the saltwater reef terms you'll meet—water chemistry, corals, equipment, and pests.
- Alkalinity
- The water's buffering capacity, the substrate corals use to build skeleton.
- Auto top-off (ATO)
- A device that replaces evaporated water to hold salinity steady.
- Calcium
- The major ion corals use, with carbonate, to build calcium-carbonate skeleton.
- Copepods
- Tiny crustaceans that form live food and part of the clean-up crew.
- Coral bleaching
- When stressed corals expel their algae and turn white.
- Cyanobacteria
- A slimy red/green bacterial film, usually a sign of excess nutrients and low flow.
- Dinoflagellates
- Stringy, bubble-laden pest algae that bloom when nutrients hit zero.
- Frag
- A small cutting of coral grown on a plug to propagate or trade.
- ICP testing
- A lab analysis that measures dozens of elements in your water.
- Kalkwasser
- A saturated lime solution that replenishes calcium and alkalinity together.
- Live rock
- Porous rock colonised by bacteria that forms the tank's biological filter.
- Magnesium
- Keeps calcium and alkalinity in solution so both stay stable.
- Nitrate
- The end product of the nitrogen cycle and the tank's main nutrient.
- Nitrogen cycle
- The bacterial process that turns toxic ammonia into safer nitrate.
- PAR
- The amount of usable light reaching corals, measured with a meter.
- pH
- How acidic or basic the water is; reef tanks sit around 7.8–8.4.
- Phosphate
- A key nutrient; too much fuels algae, zero starves corals.
- Protein skimmer
- A device that strips dissolved organic waste using a column of fine bubbles.
- Quarantine
- Isolating new fish in a separate tank to observe and treat disease.
- Refugium
- A protected sump area where macroalgae grow to export nutrients.
- RODI water
- Reverse-osmosis/deionised water — pure water for mixing salt and top-off.
- Salinity
- The dissolved-salt concentration of the water, ~35 ppt for reefs.
- Sump
- A second tank below the display that hides equipment and adds volume.
- Two-part dosing
- Dosing calcium and alkalinity from two separate solutions to keep them balanced.
- Zooxanthellae
- Symbiotic algae living in coral tissue that feed the coral via photosynthesis.