Aquarium Lighting Calculator

Aquarium lighting serves multiple purposes: it makes the tank visually beautiful, supports live plant and coral photosynthesis, and regulates the biological rhythms of fish. Choosing the right amount of light is one of the most important factors in aquarium health.

Key lighting metrics:

1. PAR (Photosynthetically Active Radiation) measures the intensity of light a plant or coral can actually photosynthesise with, in µmol/m²/s (micromoles per square metre per second). It is the only metric worth trusting for a planted or reef tank.

2. Lumens per gallon (LPG) is an older approximation, still useful for basic freshwater tanks. Roughly:

   • Fish only, no plants: 10 to 15 LPG
   • Low light plants: 10 to 20 LPG
   • Medium light plants: 20 to 40 LPG
   • High light plants: 40 to 80+ LPG

   Lumens measure brightness as a human eye perceives it, which is why the metric misleads badly on reef tanks: corals use the deep blue end that our eyes barely register, so a blue-heavy fixture reads dim and performs well.

3. Watts per gallon (WPG) was designed for fluorescent tubes and is meaningless for LEDs. You will still meet it constantly in older guides and forum posts.

Depth beats volume. This is the part most lighting guides get wrong. Light intensity falls off with the square of the distance from the fixture, so a 100 litre cube that is 30 cm deep and a 100 litre tall tank that is 60 cm deep need very different lights to put the same PAR on the substrate. Roughly four times as much, in fact, if you take the inverse-square law literally. A wide LED panel spreads better than a bare point source so the real gap is smaller, but the direction is right and the effect is large. Enter your depth above and the calculator will size it for you.

This is also why deep tanks are harder than they look, and why the plants that carpet the floor of a 30 cm tank sulk in a 60 cm one under the same fixture.

Recommended PAR levels by tank type:

Photoperiod, and why it matters: Running lights too long (14+ hours/day) causes algae outbreaks because algae can photosynthesize at lower light levels than most aquatic plants. Keep lights on for 8–10 hours for planted tanks and use a timer.

Metric and imperial:

• Tank volume in liters or gallons can be used (1 US gallon = 3.785 L)
• Tank dimensions in cm or inches both work for footprint-based calculations

LED efficiency note: Modern LED fixtures produce dramatically more PAR per watt than older T5 or T8 fluorescent tubes. The “watts per gallon” rule was designed for fluorescent lighting and significantly overestimates how much LED power you need. Always use PAR values when sizing LED fixtures.