Nominal (label) volume
The gallon size a tank is sold as. It is more than the space inside the glass, and more again than the water left once gravel, rock and equipment are in.
Actual water volume
The water your tank holds when it’s running. Work it out from the inside measurements, take off what gravel, rock and decor displace, and add any sump. Filling it with a measured bucket is more accurate than knocking off a rough percentage.
Pump output, filter circulation and maximum flow
Three different flow figures makers publish. They aren’t interchangeable, and none of them is the flow you’ll get once the filter is running in your tank. See the flow guide.
Duty cycle
The share of the day a device actually runs at full power. A heater that is on for half the day has a 50% duty cycle; its rated watts don’t change.
PAR, PPFD and watts
PAR means photosynthetically active radiation: light in the conventional 400–700 nanometer 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 meter 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 color settings, and whether the reading was taken in air or water. A center 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.