Aquarium Lighting Calculator
Enter your tank volume and what you want to grow to get a target lumen range and the right daily photoperiod, from a simple fish-only display to a high-tech CO2 planted tank.
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target lumens
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hours per day
Aim for: a light that delivers roughly over this tank. Run it on a timer for a steady schedule.
Right-sized lighting gear
Auto-matched to your tank type. Sizes are starting points; check our reviews for specific picks.
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How Much Light Does an Aquarium Need?
Lighting is one of the most over-thought and under-planned parts of a new tank. The simplest way to get in the right range is to match total lumens to what you actually want to grow. Lumens measure how bright a light looks to the human eye, and while that is not a perfect stand-in for plant energy, it is a reliable shopping guide. A fish-only display only needs enough light to see your fish clearly and bring out their color, roughly 12 lumens per gallon. Low-light plants such as java fern, anubias, and mosses thrive around 25 lumens per gallon. Moderate planted tanks want about 35, and a high-tech CO2 planted tank pushes toward 50 lumens per gallon to support fast, demanding growth. Multiply your gallons by that target and you have a working number to shop with.
Lumens Get You Close, PAR Gets You Exact
For fish-only and low-tech planted tanks, lumens are all you need to choose a fixture. Once you move into serious planted or reef territory, the precise metric is PAR, short for photosynthetically active radiation. PAR measures the slice of light energy that plants and corals actually use to photosynthesize, measured at the substrate or at the coral rather than at the surface. Two fixtures with identical lumen ratings can deliver very different PAR at depth, which is why high-tech growers and reef keepers follow the light maker PAR charts and often use a PAR meter to dial in the final result. Think of lumens as the right aisle in the store and PAR as the exact item on the shelf. If you keep demanding stem plants, carpeting plants, or corals, buy to PAR, not lumens.
Photoperiod: How Long the Light Stays On
The photoperiod is the number of hours your light runs each day, and consistency matters more than length. A fish-only or low-light planted tank is happy with 6 to 7 hours. Moderate planted tanks do well at 7 to 8 hours. A high-tech CO2 tank can run a full 8 hours because the added carbon lets plants make use of strong light. There is rarely any benefit to running longer than 8 hours, and almost every algae problem traces back to too much light for too long. The single best habit is to put your light on a timer so the schedule never drifts, plants get a steady rhythm, and you never forget to turn it off. A simple outlet timer or a fixture with a built-in scheduler does the job.
Balancing Light Against Algae
Algae is not a sign that your tank is dirty, it is a sign that light and nutrients are out of balance. The two biggest levers are the strength of your light and the length of your photoperiod. If algae is creeping in, the fix is usually to shorten the photoperiod by an hour, raise the fixture higher off the water, or dim it if it has a controller. Excess nutrients feed algae too, so balanced feeding and regular water changes go hand in hand with lighting. A light that is far stronger than your plants can use will simply grow algae no matter how clean the water is, so match the fixture to the plant load rather than buying the brightest light available. If you already own a powerful fixture, dim it, raise it, or add floating plants for shade. The aim is the right amount of light, not the most.
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Frequently Asked Questions
How many lumens does my aquarium need?
It depends on what you are growing. A fish-only tank only needs enough light to view your fish, roughly 12 lumens per gallon. Low-light plants like java fern and anubias do well around 25 lumens per gallon, moderate plants want about 35, and a high-tech CO2 planted tank wants close to 50 lumens per gallon. Multiply your gallons by that target to get a working lumen figure, then choose a light that lands in the suggested range.
Is lumens or PAR the right way to measure aquarium light?
Lumens measure brightness as the human eye sees it, which is a fine starting point for fish-only and low-tech planted tanks. For serious planted and reef tanks, PAR (photosynthetically active radiation) is the precise metric because it measures the light energy plants and corals actually use to photosynthesize. Lumens get you in the right ballpark for gear shopping, but if you run high-tech plants or corals, follow the light maker PAR charts and a PAR meter for the final tuning.
How long should I leave my aquarium light on each day?
Most freshwater tanks do well with a consistent photoperiod of 6 to 8 hours. Fish-only and low-light planted tanks are happy at 6 to 7 hours, moderate plants like 7 to 8 hours, and high-tech CO2 tanks can run a full 8 hours because the extra carbon lets plants use the light. Put the light on a timer so the schedule never changes. Longer photoperiods rarely help plants and almost always feed algae.
Why is my tank growing so much algae?
Algae thrives on a combination of too much light and excess nutrients. The most common causes are a photoperiod that runs too long, a light that is too strong for the plant load, sunlight hitting the tank, and high nitrate or phosphate from overfeeding or skipped water changes. Shorten the photoperiod, raise or dim the light, balance feeding and water changes, and make sure your plants are actually using the light you provide.
Can a light be too bright for my fish or plants?
Yes. Light that is far stronger than your plants can use simply fuels algae and can wash out shy fish that prefer dim, shaded tanks. If you have a powerful fixture over a low-tech tank, raise the light higher off the water, dim it if it has a controller, add floating or tall plants for shade, or shorten the photoperiod. The goal is to match output to what your livestock and plants need, not to flood the tank with light.
Do reef tanks use the same lighting as planted tanks?
No. Reef tanks need specialized full-spectrum LED fixtures heavy in blue and actinic wavelengths to drive coral photosynthesis and color, and they are tuned by PAR at the coral, not by lumens. Planted freshwater tanks want a broader white spectrum that favors plant growth and natural color. Always buy a fixture built for your tank type, a reef LED for corals and a planted or full-spectrum LED for plants.