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Aquarium lighting: intensity, spectrum, and day length are three separate decisions

Light serves you, the plants and the algae at once. Decide who it's for before you buy.

2 min readUpdated September 25, 2026

About this guide

Environment

By Fishician

Published September 8, 2026

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The short answer

  • Fish-only tanks light for viewing. Fish need a steady day-night cycle and shade, not intensity.
  • Plants and coral use light as energy, but only with carbon and nutrients to match. Extra light alone often grows algae.
  • Three separate dials: intensity, spectrum and duration. They aren't interchangeable.
  • Oxygen swings with the light. In planted or algae-rich tanks, oxygen often rises during light and falls overnight. Aeration and other factors also matter.
  • A timer beats a fancy fixture. Consistency is what fish and plants respond to.
An LED aquarium light mounted above a planted tank
A real LED fixture above an aquarium. Light intensity and daily duration both influence the tank.Photo: Led Aquarium Light — Lime Huang · CC BY-SA 3.0. Resized for display.
The three lighting dials
Intensity
  • Energy reaching a point
  • Falls with depth and haze
  • A rating means little without its depth
Spectrum
  • Which wavelengths
  • Box names aren't standard
  • Trust published spectral data
Duration
  • Hours on per day
  • Cheapest to change
  • Change one way, then wait

Before buying a fixture

  • Know the tank's depth and footprint, and what in it is photosynthetic.
  • Check published output and spectral data, mounting distance and ventilation.
  • Plan how it will be controlled: timer or controller.

Changing the photoperiod

  • There's no universal right number of hours.
  • Change duration in one direction, note the date, and hold it long enough to see the effect on plants, algae and fish.
  • Light is only one algae input. Cutting it doesn't remove built-up nutrients and can starve your plants.

Signs light is part of a problem

  • Algae speeding up on lit surfaces.
  • Plants thinning despite enough light (often missing carbon or nutrients).
  • Fish breathing hard at the surface early in the morning. Add surface movement. See aeration.
  • Temperature rising with the lighting schedule.

Sunlight and safety

  • Window light is an uncontrolled extra photoperiod that also adds heat. Move or shade the tank.
  • Drip loops on cords, connections above the water line, and fixtures rated for the humidity.

Who the light is for

  • Fish only: the lowest intensity that shows the fish clearly, plus shaded cover.
  • Planted: light matched to the carbon and nutrients available.
  • Reef: intensity and spectrum matched to the corals you keep.
  • A fixture chosen for one of these is often wrong for another.
  • Record every change to a lighting setting with its date.

Sources

Each source is listed with the part of this guide it supports.