Nominal volume
The size label used to name an aquarium. It is different from the inside geometric water column and from actual working water volume after displacement.
Working water volume
The water present at normal operation. Estimate it from internal dimensions and account for displacement and any connected sump. A measured fill or displacement is stronger than an unexplained percentage deduction.
Pump output, filter circulation and maximum flow
Different manufacturer flow labels that should retain their definitions. They are not interchangeable and do not automatically describe delivered flow through an installed aquarium. See the flow guide.
Duty cycle
The share of time equipment is actively operating at its stated load. A heater running half the day has a 50% duty assumption; that does not mean its rated watts have changed.
PAR, PPFD and watts
PAR means photosynthetically active radiation: light in the conventional 400–700 nanometre wavelength band. A chart’s “PAR” reading usually means PPFD, photosynthetic photon flux density. It counts photons in that band reaching a surface each second, reported in micromoles per square metre per second (µmol/m²/s). Apogee’s quantum-meter manual explains the measurement.
Read the chart’s distance carefully: distance below the lamp is different from depth below the waterline. Match the measurement position under the fixture, brightness and colour settings, and whether the reading was taken in air or water. A centre reading does not describe every corner. Underwater readings need the sensor’s appropriate calibration or correction; Apogee explains why an air-calibrated sensor can read low underwater. Do not extend a chart’s value to an unmeasured depth.
Electrical watts describe power consumption, not light reaching plants or plant suitability. For the separate mounting and control checks, read the planted-light fit guide.
Manufacturer-rated candidate
A product inside the specified maker rating used for a shortlist. Physical fit, installation, intended livestock and other requirements remain separate checks. This is not a universal compatibility verdict.
Unknown
A fact that the inspected source does not supply or that remains unresolved. Unknown values never turn into zero, a positive recommendation or an invented performance estimate.