How Long Should You Run Your Aquarium Light?

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.