Aquarium lighting runs on a simple trade: plants and viewing pleasure on one side, algae and tired fish on the other. The number most tanks thrive on is six to eight hours a day, but the hours themselves are only half the answer — a tank lit for seven erratic hours fares worse than one lit for eight hours like clockwork. Fish, plants and algae all read the light as a calendar, so your job is to publish a reliable one.
The simple answer: six to eight hours for most tanks
A planted tank with easy, low-demand plants does well at six to seven hours; a brighter, more densely planted setup can use the full eight. Fish-only tanks need light mainly so you can see the fish and so they can live by a day rhythm, and five to seven hours is plenty — the fish get ambient room light for the rest of their waking day. New tanks earn a shorter start: begin around five or six hours while plants establish and the system stabilises, because a young tank under a long photoperiod is an algae farm with fish in it. Extend by half an hour a week only if the plants ask for it with pale, reaching growth.
Why fish need a real night
Fish have no eyelids and no say in the matter: when the light is on, their day is on. A proper dark period — genuinely dark, not a blue glow left burning until midnight — is when fish rest, when shy species venture out to feed in the dusk, and when the tank's whole metabolism resets. Lights left on around the clock stress fish visibly: colours wash out, appetites wobble and algae rejoice. A short blue-light viewing window in the evening does no harm, but treat it as a nightlight with an off time, not a second shift. Sudden switching startles fish too, so where you can, let room light come up before the tank light fires, or use a light with a gradual ramp.
Timers, and algae as the warning light
Put the light on a timer today if it is not already. The timer removes the two classic errors — the forgotten Sunday lie-in with the tank blazing since dawn, and the evening the light never came on at all — and it lets you place the lit hours when you are home to enjoy them, which for most working keepers means a late-morning start and an evening finish. Read algae as the tank's own report on your schedule: green film racing across the glass or hair algae in the plants almost always means the photoperiod is too long for the nutrient load, and the first correction is to cut an hour of light, not to reach for chemicals or new gadgets.
Adjusting when plants, fish or algae disagree
Change one thing at a time and give it a fortnight: shorten or lengthen by half an hour, then watch plant growth, fish behaviour at lights-on and the pace at which the glass greens up. Some keepers split the day — four hours on, a midday break, four more on — which can inconvenience algae while totalling the same plant hours; it works for some tanks and is worth trying if a straight block keeps failing you. Note what you changed and what followed, because lighting problems are diagnosed from the pattern, not from any single day. If a shorter day and hungry-looking plants pull you both ways, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.