How long to leave aquarium lights on: photoperiod and intensity for plants

How many hours

For a new tank, run 6 hours a day for the first two or three weeks, then add 30 minutes a week until you reach 7 or 8. Young plants cannot use much light yet, and whatever they leave goes to algae.

• low-tech tank, no CO₂, modest light: 6–8 hours,
• strong light with injected CO₂: also 6–8 hours – the stronger the light, the more reason to stay at the short end,
• longer than 8 hours: only in an established tank where plants are growing well and algae is not spreading; Tropica advises against more than 10 hours a day – longer lighting adds nothing for the plants and plays into the hands of algae.

If algae picks up while you are ramping, stop adding time and hold where you are.

Hours are only half of it: intensity

What plants actually receive is a daily dose of light – intensity multiplied by hours. Weaker light for 8 hours and strong light for 6 can add up to a similar dose. Longer hours only partly make up for a weak fixture, though: below a certain intensity plants barely grow however long the light is on. The fix is then a stronger fixture or less demanding plants.

Light for plants is given as PAR (strictly, PPFD), in µmol/m²/s. There is no agreed scale; the split most often quoted in the hobby is by PAR at the substrate:

• up to about 30: low light – enough for anubias, ferns, crypts, bucephalandra and mosses,
• 30–50: medium light – most stem plants, and CO₂ starts to pay off,
• above 50: high light – demanding and red plants, and without CO₂ expect algae.

Other authors and fixture makers quote much higher numbers because they measure closer to the light. PAR drops quickly with distance from the fixture, so a reading taken just below the light is far higher than what plants get at the bottom of a deeper tank.

Setting up the daily schedule

Run the light as one continuous block on a timer, same time every day. Switching it by hand whenever you remember changes the dose from day to day.

If your fixture has a sunrise and sunset ramp, use it. A light snapping on in a dark room startles fish. Count the photoperiod from lights-on to lights-off, ramps included – that is also how SUISO counts it in the light schedule.

A midday break (a “siesta”) is popular in low-tech tanks, but there is no solid evidence that it prevents algae, so you do not need one. It is mainly useful if you want the tank lit both before work and in the evening.

Daylight from a window adds to the dose. Keep direct sun off the tank: it feeds algae and heats the water.

Light and CO₂ go together

The stronger the light, the more CO₂ and nutrients plants need to use it. When those run short, algae takes the surplus light. That is why strong light without injected CO₂ almost always ends in algae, and why low-tech tanks are better off with gentler light.

Turn CO₂ on one to two hours before the lights so the level is already on target when they come on, and switch it off about an hour before lights-out, at the latest with the lights – plants do not use it in the dark. Aim for 20–30 mg/l (ppm); more than that puts fish and shrimp at risk. If fish gasp at the surface, turn the CO₂ off and break the surface. An air stone overnight helps because plants and fish only consume oxygen in the dark.

Too little or too much light

• stretched stems with long gaps between leaves – too little light,
• red plants turning green or staying orange – usually too little light, though CO₂ and fertilizer play a part,
• bare lower stems – often shading from the top growth or a lack of CO₂ rather than a weak fixture; trim and replant the tops,
• algae growing faster than the plants – more light than the CO₂ and nutrients can match.

When algae shows up, cut the photoperiod to 6 hours first and then look for other causes. Floating plants shade the tank and soak up nutrients at the same time; if they shade the plants below too much, take some out.

Common mistakes

• running 10–12 hours “to make plants grow faster”,
• a spot where direct sun hits the tank,
• a stronger light without adjusting CO₂ and fertilizer,
• changing hours and intensity at the same time – or together with fertilizer,
• no timer.

After any lighting change, give it at least a week before judging the result. Plants and algae both respond with a delay.

What to log

On and off times, the dimmer setting, a new fixture and every time you lengthen or shorten the photoperiod. Keep the fixture in your equipment list so you can see when it went in. When algae appears or stems start to stretch, you can see whether the light changed just before – one of the first things worth checking. The Advisor reads the light schedule from your tank settings, so it takes it into account when you ask about algae.