How Long Should You Run Your Aquarium Light?
Too long and you grow algae; too short and plants suffer. The right photoperiod for a planted or fish-only tank, and how to use a timer.
Run a planted tank light for about 6 to 8 hours a day, and a fish-only tank for even less, around 6 hours, purely for viewing. Longer photoperiods do not help plants past a point and mainly feed algae. Put the light on a timer so the schedule is identical every day, since consistency matters more than the exact hour. If algae appears, cut the time back before you change anything else.
More light is not better light. The number of hours you run the fixture (the photoperiod) is one of the biggest levers you have over algae, and it costs nothing to get right.
The working numbers
- Planted tank: 6 to 8 hours a day. Most low-tech tanks are happiest around 7.
- Fish-only tank: about 6 hours, and only for viewing, since the fish do not need it. A fish-only tank can even skip the light entirely.
- High-tech, high-light, CO2 tank: often 6 hours or split into two blocks, kept short precisely because the intense light feeds algae fast.
Why longer is not better
Plants can only use so much light per day; past their fill, extra hours do nothing for them and everything for algae, which is delighted to photosynthesize in the extra time. If you are fighting algae, the first and cheapest fix is almost always to shorten the photoperiod by an hour or two, not to add chemicals.
Put it on a timer
The single best upgrade here is a cheap outlet timer, or the built-in scheduler on a light like the programmable ones in our planted light guide. Fish and plants both thrive on a predictable day and night, and a timer means you never leave the light on for twelve hours by accident, which is a classic algae trigger.
The siesta method
If algae persists at a normal photoperiod, some aquarists split the light into two blocks with a dark break in the middle (say four hours on, three off, three on). Plants keep enough light while algae, which ramps up more slowly, loses momentum during the break. It is worth trying before spending on anything.
Pair the schedule with the right fixture from the aquarium light rankings, and if your new tank is cloudy rather than algae-green, that is a different problem covered in why is my new aquarium cloudy.
Start at 7 hours a day on a timer and adjust from there: less if algae shows up, a touch more only if healthy plants look starved. A consistent, moderate photoperiod grows plants and starves algae better than a long or erratic one.
Common questions
How many hours should aquarium lights be on?
About 6 to 8 hours a day for a planted tank, and around 6 for a fish-only tank kept purely for viewing. Longer does not help plants past a point and mainly feeds algae, so a timer keeps it consistent.
Will a long light period cause algae?
Yes, it is a leading cause. Light beyond what the plants use fuels algae, so shortening the photoperiod is the first fix to try when algae appears, ahead of buying anything.
Should I put my aquarium light on a timer?
Yes. A timer gives plants a consistent day-night rhythm and stops the light being left on too long, which is the most common algae trigger. A basic plug-in timer is inexpensive and works with any fixture.