Light is food for plants and a clock for fish, so getting it roughly right matters more than buying the brightest lamp in the shop. Too little light and plants stall, melt, and lose the race against algae. Too much light, or light left on all day, feeds algae instead. This free calculator turns your tank size and plant level into a wattage guide and a daily photoperiod, so you can set a timer once and leave it alone.
How to use this calculator
Enter your tank size in litres and pick your plant level: low, medium, or high. Low means hardy plants like Java fern and Anubias. Medium means most stem plants and carpets that grow steadily. High means dense, fast-growing planted tanks, often with added carbon dioxide. Press calculate and the tool shows watts and hours per day. Use those numbers to pick a lamp and set your timer.
How the calculation works
The tool uses watts per litre as a simple guide: about 0.25 watts per litre for low light, 0.5 for medium, and 0.8 for high light tanks. The photoperiod follows the same ladder, at 6, 8, and 10 hours a day. These are starting points for modern LED lamps, and older lamp ratings do not compare directly, since LEDs give more light per watt. If your plants stretch tall and pale, they usually want more light, not more fertiliser.
A worked example
A 100 litre tank with medium light plants needs about 50 watts and 8 hours of light a day. The same tank with low light plants needs only about 25 watts and 6 hours, while a high light planted tank needs about 80 watts and 10 hours. If your lamp is stronger than the guide, shorten the hours first instead of dimming the plants into darkness, since a steady short day beats a long dim day for most tanks.
Reading your result
Treat watts as a shopping guide and hours as the habit that matters most. A timer is the cheapest upgrade in this hobby, since fish and plants both do better on a steady day and night cycle. If algae appears, cut the hours by one or two before you buy chemicals, and check whether direct sun hits the tank. If plants still struggle after two weeks, look at nutrients and carbon dioxide before adding more light.
Tips for healthy plants
Start with the hours in your result and raise them slowly, one hour at a time, over a few weeks. Keep the glass clean so light actually reaches the plants. Trim fast growers before they shade everything below them. Floating plants are a useful dimmer switch, since you can thin them out to let more light through. Remember that new plants often melt old leaves first and then grow fresh ones suited to your tank.
Common mistakes
The biggest mistake is leaving lights on for 14 hours or more, which almost always grows algae. The second is placing the tank in direct sun and then blaming the lamp. The third is buying a very strong lamp for low light plants and wondering why algae wins. Match the light to the plants you actually keep, keep the day steady, and change only one thing at a time so you can see what helped.
Frequently asked questions
Do fish need light at night? No, fish need a dark night just like plants do, so switch the lamp off and let the room go dark. Does a blue moonlight count as night? A very dim blue light for an hour or two in the evening is fine for viewing, but it should not run all night. How long should a planted tank light stay on? Most tanks do well on 6 to 10 hours, and this calculator picks a point in that range for your plant level.
A note on algae
Algae is not a sign that your tank is dirty or that you failed. It is a sign that light, nutrients, and plant growth are out of balance. Cut light hours first, feed a little less, change water weekly, and let plants fill in. Fast-growing stem plants and floating plants compete with algae for the same nutrients, which is why a well-planted tank often clears up on its own once the plants settle.
Versión en español: Calculadora de iluminación para acuarios – Vatios y horas según plantas.