Planted Tank Lighting: Matching the Light Schedule to Your CO2

Quick answer

In a planted tank, light is the accelerator and carbon dioxide is the fuel. Plants only use as much light as they have CO2 and nutrients to match. Give them more light than the CO2 can support and the surplus does not grow more plants; it grows algae.

So the light schedule is not set in isolation. You match the intensity and the daily hours to your CO2 level. A tank with pressurised CO2 can run bright and long and grow fast. A tank with no added CO2 needs modest light and a modest photoperiod, or algae wins.

• Light drives photosynthesis, but CO2 or nutrients usually set the real speed limit.
• Excess light beyond what CO2 supports feeds algae, not plants.
• No added CO2 means keep light moderate and the photoperiod shorter.
• Injected CO2 lets you run more light and a longer photoperiod safely.
• Algae is feedback: an outbreak usually means light and CO2 are out of balance.

Decide in 60 seconds

Why light and CO2 must match

Photosynthesis needs three things at once: light energy, carbon dioxide as the carbon source, and nutrients. A plant can only build tissue as fast as the scarcest of those allows. That scarcest ingredient is the limiting factor, and in most tanks it is carbon.

Water holds far less available CO2 than air, and a tank without injection is running on the trickle that dissolves from the surface and from fish respiration. That trickle caps how fast plants can grow, no matter how bright the light.

Now add more light on top of that cap. The plants cannot use it, because they have no more carbon to work with. Algae, which are far less fussy and thrive on light and dissolved nutrients, take up the excess instead. This is why a common cause of an algae outbreak is simply too much light for the tank's carbon supply.

Matching solves it. Either keep the light within what your CO2 can support, or raise the CO2 to meet a brighter light. Both are valid; running high light with low carbon is the combination that fails.

Setting the photoperiod

Consistency is the foundation. Put the lights on a timer so they come on and go off at the same times every day, and keep the tank out of direct sunlight, which is uncontrolled extra light that feeds algae.

Many planted tanks run somewhere around six to eight hours of light a day, and that is a sensible starting band rather than a rule. The right length depends on your light's intensity and your CO2, so treat it as a dial to adjust based on how the tank responds.

Fish also need a genuine dark period to rest, so a real night matters as well. Do not leave lights on around the clock, and do not rely on room light as a substitute for a proper off period.

On a new tank, start at the shorter end. Young plants take time to establish, and a long, bright photoperiod before they are growing well just hands the advantage to algae. Extend the hours gradually as the plants fill in and start to pearl.

Matching light to your CO2 level

Low-tech tanks, with no injected CO2.

Keep the light modest and the photoperiod on the shorter side. Choose plants that tolerate low carbon, accept slower growth, and resist the urge to fix a sparse-looking tank with more or longer light. The carbon supply is your ceiling, and pushing light past it produces algae, not lushness.

Injected CO2 tanks.

Pressurised CO2 raises the carbon ceiling, so plants can use brighter light and a longer day, and grow much faster. Run the CO2 on a timer through the lit hours only, and switch it off at night, when plants stop consuming it and it would otherwise build up. Many keepers start the CO2 a little before lights-on so a good level is present when the light arrives.

Changing from one to the other.

If you increase light, increase the carbon to match, whether that means adding CO2 or accepting you must keep light lower. Raising light alone is the classic misstep.

Reading algae as feedback

Treat algae as a gauge, not just a nuisance. A persistent outbreak is the tank telling you the light is out of step with the carbon and nutrients.

The usual first move is to reduce, not add: shorten the photoperiod or lower the intensity so the plants are no longer being over-lit for their carbon supply. On an injected tank, the alternative is to raise CO2 and nutrients so the plants can actually use the light they are getting.

Give any change time. Plants and algae both respond over days to weeks, not hours, so change one thing, hold it, and watch before changing again.

:::danger
Too much dissolved CO2 will suffocate your fish.

CO2 injection lowers the water's pH and directly reduces the oxygen fish can use, and an overdose can kill a whole tank. If fish gather at the surface gasping during the lit hours, turn the CO2 off at once, increase surface movement and aeration, and reduce the injection rate before running it again. Always put CO2 on a timer that shuts it off at night.
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What never to do

Do not run more or brighter light hoping to grow more plants when your CO2 is limited. You will grow algae instead.

Do not leave aquarium lights on continuously. Plants and fish both need a dark period, and round-the-clock light drives algae.

Do not leave CO2 injecting overnight. Plants stop using it in the dark, so it accumulates and can suffocate the fish.

Do not place the tank in direct sun and then try to control algae with the timer. The sunlight overrides everything.

Do not chase fast fixes. Change one factor, hold it, and judge the result over days.

When to get help

The urgent case here is the fish, not the plants. If fish gasp at the surface during the day while CO2 is running, treat it as a CO2 overdose: turn the gas off, boost surface agitation, and reduce the rate. Persistent or severe distress warrants aquatic-vet advice.

For stubborn algae or failing plants that will not respond to rebalancing light and CO2, an experienced planted-tank keeper or aquatic specialist can help read the whole system. Have your light type and hours, whether you inject CO2, your fertiliser routine, and your water parameters ready, since the fix is almost always in that balance.