Light requirements for a planted nano tank
Plants are specified in PAR. Lights are specified in watts and lumens. Nobody publishes the conversion, and this page is about working sensibly inside that gap.
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The units problem
PAR - photosynthetically active radiation - is what plants actually use, measured at a given depth. Aquarium Co-Op publishes Anubias Nana Petite at 10-20 PAR. That is the number that decides whether a plant grows.
Lumens measure brightness weighted for human vision. Fluval publishes 821 lm for the Spec V’s light. Watts measure power consumption, which across different LED efficiencies tells you almost nothing about output.
There is no published conversion between them. It depends on spectrum, beam angle, mounting height, water depth, water clarity and tank geometry. A conversion factor would be a guess with arithmetic wrapped round it, so we do not publish one.
What to do instead
Work in classes rather than numbers.
| Class | Roughly | Plants | CO2 |
|---|---|---|---|
| Low light | 10-30 PAR at substrate | Anubias, java fern, crypts, mosses | Not needed |
| Medium | 30-50 PAR | Most stem plants, easier carpets | Helpful |
| High | 50+ PAR | Demanding carpets, red plants | Required |
Every plant this site recommends is in the first row. Any nano fixture sold for planted tanks - including the 6.5W Fluval C.O.B. and every kit light in the tanks roundup - clears it comfortably. In a betta tank, light output is genuinely not your constraint.
Depth matters more than most people expect
Light falls off sharply through water. That is a real advantage of a nano tank: the substrate of a 12-inch-tall tank receives a great deal more of the fixture’s output than the substrate of a 24-inch one. It is one of the few things that is easier at small scale.
It also means a fixture mounted on tall legs above an open-top tank delivers less to the substrate than one sitting on the rim. If plants at the bottom are struggling, lowering the light is free and is worth trying before buying anything.
Photoperiod beats output
This is the part that actually decides whether your tank looks good.
Eight hours a day, on a timer. Plants photosynthesize for a limited window and then stop; algae, which is more opportunistic, keeps taking advantage of whatever light remains. A long photoperiod hands the advantage to algae without helping the plants.
A siesta split - four on, three off, four on - suits a tank you want lit morning and evening. Plants tolerate the interruption; algae, which ramps up more slowly, is disadvantaged by it.
Consistency matters more than the exact figure. A tank that gets eight hours every day at the same time is more stable than one that gets six on some days and twelve on others.
If you have algae
Reduce the photoperiod before you change anything else. Six hours for two weeks costs nothing and resolves a large proportion of algae problems in low-tech nano tanks. A brighter light and a longer day is the usual instinct and it is backwards.
If a fixture gives you color channels - the Plant 3.0 Nano publishes six independently controlled band waves - turn the blue down as well. Blue drives algae hardest.
Sunlight is not a light source
A tank on a windowsill receives far more light than any aquarium fixture, entirely uncontrolled, varying by season and weather. It will grow algae faster than you can remove it, and it will swing temperature. Site the tank away from direct sun - which is a decision you make before you fill it, because a filled tank does not move.
What we cannot tell you
The PAR any specific fixture delivers at your substrate. Nobody publishes it, we have no meter, and estimating it from wattage and lumens across different LED densities and beam angles would be a fabrication with a decimal point on it. The class-based approach above is the honest way to work without that number.
What we would buy for this
Chosen from the same registry as our roundups, on published specifications and the arithmetic they imply - not on testing we have not done.
Fluval C.O.B. Nano LED, 6.5W
Six and a half watts, no app, no schedule, no color channels. For a low-light betta tank growing anubias and java fern that is not a compromise, it is the correct specification - and it costs a fraction of the programmable option. Put it on a mechanical outlet timer and you have a photoperiod.
Skip it if: You are growing carpeting plants or anything Aquarium Co-Op rates above low light. Six and a half watts will not do it.
Fluval Plant 3.0 Nano Bluetooth LED, 15W
Fifteen watts of programmable full-spectrum light over a nano tank, with a 24-hour ramp you set in an app and six independently controlled band waves. It is more light than a low-tech betta tank needs and exactly the right light for a planted one. The IP67 rating is the underrated part: it can sit directly over open water without a lid.
Skip it if: You will not use a phone to run a light. There are no physical controls at all - Fluval states it is operated exclusively via the app.
Frequently asked
How much light does a planted nano tank need?
For low-light plants - which is everything this site recommends - any nano fixture sold for planted tanks is sufficient. Aquarium Co-Op publishes anubias at 10-20 PAR, and that is a low bar at nano depth.
What is PAR and why does it matter?
Photosynthetically active radiation: the part of the light spectrum plants can actually use, measured at a depth. It matters because lumens measure brightness as a human eye sees it, which is not the same thing - and because plant care data is published in PAR while fixtures are not.
Can I use a regular LED lamp over an aquarium?
Physically yes, and there are two real problems: household lamps are not waterproof, which matters over an open tank, and their spectrum is chosen for how a room looks rather than for plant growth. An aquarium fixture with an IP rating - Fluval publishes IP67 for the Plant 3.0 Nano - is the safer choice.
Why is my low-light plant covered in algae?
Usually too much light for too long, especially on slow growers like anubias whose leaves stay in place for months and accumulate anything that lands on them. Cut the photoperiod to six hours, reduce blue if you can, and consider floating plants to shade the tank.