Start with the number plants can use
My earliest aquarium light was selected by watts per gallon, a rule that survived because old fluorescent tubes were similar enough to make it feel predictive. LEDs ended that convenience. A 20-watt fixture can be efficient or poor, narrow or broad, close to the surface or suspended high. Electrical consumption tells you what the lamp draws from the wall, not what the foreground receives.
Plants use photons in the photosynthetically active range, conventionally 400–700 nanometres. PAR names that range; PPFD measures the number of those photons arriving at a square metre each second, usually in micromoles per square metre per second. For aquariums, a PAR meter reading at the substrate or plant canopy is far more informative than fixture wattage.
Lumens weight light according to human visual sensitivity. They are useful for comparing how bright lamps look to us, but two fixtures with the same lumens can deliver different photon distributions to plants. Kelvin describes the visual character of white light, from warm to cool. It is not an intensity rating. A 6500 K label does not certify plant-growing power any more than paint color certifies the strength of a wall.
The aquarium changes the light after it leaves the fixture
Intensity falls with distance and with the spreading geometry of the source. The simple inverse-square rule describes a point source in open space, but an aquarium fixture is an extended array close to glass and water, so real falloff is not that tidy. Still, the practical direction is reliable: raise a light and intensity at the substrate drops while spread usually becomes more even.
Water itself absorbs and scatters light, and dissolved color, suspended particles, surface agitation, glass lids, hardscape, and plant leaves remove more. A PAR map is therefore a property of the whole setup, not the model name. The same fixture can create high light in a shallow rimless tank and modest light at the bottom of a deep aquarium with floating plants.
Measure in several places if you can borrow a meter. The center often receives much more than the corners, while a hardscape shadow can create a low-light refuge centimetres from a bright carpet. If measurement is unavailable, begin lower than the fixture’s maximum, watch the form of new growth, and change one variable at a time.
Spectrum matters, but not in the way color charts imply
Chlorophyll absorbs strongly in blue and red regions, which has encouraged fixtures that look magenta and marketing diagrams that imply green light is wasted. Leaves absorb green less strongly near the surface, but it penetrates deeper into tissue and plant canopies. Broad white light can grow plants very well while rendering the aquarium in a way people actually enjoy.
Spectrum influences appearance, pigmentation, and aspects of plant form. Red plants often look richer under a spectrum with good red rendering, even before biology changes. Some species alter morphology in response to spectral cues. But spectrum cannot rescue insufficient intensity, and a supposedly perfect spectrum cannot compensate for missing carbon or nutrients.
Choose color quality for a truthful, pleasing view, then manage usable intensity. Avoid buying solely by an exaggerated spectral graph without a scale. Independent PAR maps, fixture dimensions, mounting height, and the tank’s depth tell you more about likely performance than a row of colored peaks.
Photoperiod is a dose, not a substitute for balance
Intensity and duration combine into a daily light dose. Extending a weak light can add photons, but photosynthesis does not respond linearly forever, and a long dim day is not identical to a shorter useful one. For a new planted tank I usually begin around six hours. Once plants are rooted and the system is stable, seven or eight may suit the display and the species. Twelve hours is rarely the gentle default people imagine.
More light raises the demand for inorganic carbon and mineral nutrition. Underwater carbon supply is unusually constrained because gases diffuse far more slowly in water than air. When lighting increases while CO₂, flow, or nutrients do not, plants may stall even as opportunistic algae exploit the instability. Turning up the light is therefore a system adjustment, not a cosmetic one.
Use a timer and keep the schedule boring. Judge changes by new leaves, internode spacing, growth direction, and algae over at least a couple of weeks. If stems stretch and lower leaves disappear, intensity or distribution may be inadequate. If growth is dense but algae appears after an intensity increase, restore the previous setting before adding three new remedies. Aim for the quietest setting that still gives you the plant form you want; the brightest number the fixture can reach is rarely the answer.
Light sets the plant’s appetite. It does not supply the carbon and minerals needed to satisfy it.