Knowledge Centre · Planted aquariums
Aquarium lighting for planted tanks: PAR, photoperiod and spectrum
Light is the single input every planted tank needs and the one most commonly mismatched to what's actually being grown. The number that matters is PAR — the usable light reaching your plants — not the wattage printed on a box, and matching it to your plants' actual demand tier prevents two separate, opposite failures: undemanding plants surviving but never thriving under too little light, and excess light outrunning your CO2 and nutrients, which is one of the most common causes of algae in an otherwise healthy tank. This guide covers PAR and light tiers, photoperiod, spectrum, and how lighting decisions connect to the rest of a planted setup.
Before you choose a light
This guide helps if…
You're choosing a light for a new planted tank, or your current plants are struggling and you suspect lighting — too little, too much, or the wrong duration — is part of the cause.
Look elsewhere first if…
You haven't yet decided what plants you want to keep — see our best beginner aquarium plants guide first, since plant choice should generally drive light choice, not the other way round.
Worth knowing: lighting, CO2 and fertilising work as a set, not independently. Increasing light without also increasing CO2 and nutrient availability doesn't grow plants faster — it mainly shifts the balance toward algae. See our CO2 guide and fertilising guide for the other two legs of that balance.
PAR vs watts and lumens
PAR (photosynthetically active radiation) measures the portion of light plants can actually use for photosynthesis, at a specific distance from the light source — it's a measurement of usable intensity where your plants actually sit, not a property of the fixture alone. Wattage measures electrical draw, and lumens measure brightness as perceived by the human eye; neither reliably predicts how much usable light reaches your substrate, since efficiency varies hugely between bulb types and individual fixtures[1]. Two lights of identical wattage, one an older fluorescent tube and one a modern LED, can produce very different PAR at the same depth.
If a manufacturer publishes a PAR chart or PAR-at-depth figures for a fixture, that's meaningfully more useful for planning than wattage or lumens alone. Where one isn't available, the light tiers below (grounded in commonly cited PAR ranges) are a reasonable way to reason about a fixture's likely output relative to your plants' needs, without needing to buy a PAR meter yourself.
Low, medium and high light tiers
| Tier | Typical PAR at substrate | Suits |
|---|---|---|
| Low | Roughly 20–40 µmol/m²/s | Anubias, Java fern, Cryptocoryne, most mosses — undemanding, shade-tolerant plants |
| Medium | Roughly 40–80 µmol/m²/s | Amazon sword, Vallisneria, most stem plants — healthy growth without CO2 injection being strictly required |
| High | Roughly 80–150+ µmol/m²/s | Carpeting plants and demanding red/coloured stems — generally needs supplemental CO2 to avoid an algae-prone imbalance |
These bands are widely used starting points from specialist aquascaping sources rather than a fixed scientific standard[1][2], and different sources draw the boundaries slightly differently. Treat them as a general sense of where a fixture and plant combination sits, not a precise cutoff to hit exactly.
Matching light to your plants
Choose your light tier around your most demanding plant, not your least demanding one — low-light plants generally tolerate brighter conditions than their minimum without harm, but high-light plants placed under lighting sized for low-light species usually won't thrive regardless of fertilising or CO2. If your tank mixes tiers (a common carpeting-plant-plus-Anubias combination, for example), it's usually more practical to light for the demanding carpet and let the shade-tolerant plants sit lower or in a shadier spot than to under-light the carpet to protect the Anubias. See our plant comparison guide for which common species fall into which tier.
Choosing a photoperiod
Roughly 6–8 hours a day is a widely used starting point for a newly set up planted tank[3], adjusted from there based on how plants and algae actually respond rather than fixed permanently. A longer photoperiod doesn't reliably speed growth once total daily light is already sufficient for your plants' CO2 and nutrient supply to keep up with — it mainly extends the window during which algae can also use that light. If algae becomes a recurring problem, reducing photoperiod by an hour or so, alongside checking dosing and CO2, is a more useful first response than adding more light in the hope of "outcompeting" it.
Spectrum: does colour temperature matter?
Plants primarily use light in the red and blue portions of the visible spectrum for photosynthesis, but in practice, most modern full-spectrum aquarium LEDs marketed for planted tanks provide enough of both to support healthy growth without needing to chase a specific colour-temperature number. Spectrum affects how plants and substrate look to the eye — warmer or cooler-toned lights change the tank's visual appearance — more than it changes whether plants can photosynthesise adequately, provided the fixture is a genuine full-spectrum plant light rather than one built purely for fish-viewing colour enhancement.
Why too much light causes algae, not faster growth
Photosynthesis needs light, CO2 and nutrients together; supplying more of one input beyond what the other two can support doesn't speed growth — it just leaves a surplus. Algae grows faster than most aquarium plants and is well placed to exploit exactly that kind of surplus[2], which is why an intense light on a tank without matching CO2 and dosing is one of the more common, avoidable causes of algae outbreaks. See our algae in aquariums guide for the broader picture beyond planted-tank-specific causes, and our nutrient deficiency guide if plants themselves look unhealthy alongside the algae.
Depth, spread and fixture placement
PAR falls off substantially with depth and distance from the light source, so a fixture that comfortably lights a shallow tank may under-light a taller one even at identical wattage or rated PAR-at-the-glass. Check a fixture's PAR-at-depth data against your actual tank height where it's available, and be aware that fixtures mounted higher above an open-top tank, or dimmed for aesthetic reasons, deliver less usable PAR at substrate level than the same fixture run at full intensity close to the water's surface.
Common mistakes
- Choosing a light by wattage or lumens alone, rather than checking PAR-at-depth data where it's available.
- Running a long photoperiod from day one before plants, CO2 and dosing have caught up with the light being supplied.
- Responding to algae by adding more light, rather than reducing photoperiod or intensity and checking CO2/dosing balance.
- Under-lighting a demanding carpet plant to protect low-light species in the same tank, when the reverse arrangement generally works better.
- Ignoring tank depth when comparing PAR figures between fixtures rated at the glass rather than at your actual substrate depth.
Frequently asked questions
What is PAR and why does it matter more than watts?
PAR (photosynthetically active radiation) measures the light actually usable for photosynthesis in the 400–700 nanometre range, at a given distance from the light source. Wattage only tells you how much electricity a fixture draws, which varies hugely between old fluorescent tubes and modern LEDs — two lights with identical wattage can produce very different PAR. If a fixture's manufacturer publishes a PAR chart at different depths, that's a far more useful figure than wattage for judging whether it suits your tank and plants.
How many hours a day should planted tank lights be on?
Roughly 6–8 hours a day is a widely used starting point, adjusted from there based on plant growth and algae rather than fixed permanently. A longer photoperiod doesn't reliably grow plants faster once you're already supplying enough total light — it mainly increases the risk of feeding algae with light your plants and nutrients can't fully use.
Is more light always better for plant growth?
No — this is one of the most common planted-tank misconceptions. Light drives photosynthesis, but photosynthesis also depends on CO2 and nutrients being available to keep pace. Light beyond what your CO2 and fertilising can support doesn't speed growth further; it mainly creates a surplus that algae, which grows faster than most aquarium plants, is well placed to exploit.
Can I keep low-light and high-light plants in the same tank?
Within limits. Low-light plants like Anubias and Java fern generally tolerate brighter light than their minimum without harm, so a tank lit for medium-light stem plants can usually still host them — placed lower in the tank or in a shaded spot if the light is genuinely intense. The harder direction is the reverse: high-light, demanding plants placed under lighting sized only for low-light species usually won't thrive, regardless of dosing or CO2.
How AquaPulse can help
Lighting problems in a planted tank rarely announce themselves the day they start — algae builds gradually, and a slowly declining carpet plant looks similar to a healthy one for longer than most keepers expect. Working out whether last month's light or photoperiod change actually caused this month's algae depends on being able to compare what's happening now against what your tank looked like before you changed anything, not on memory alone.
AquaPulse works from the information and context you enter — it doesn't measure your light output automatically. Logging notes alongside your regular water testing in AquaPulse keeps a record of changes like a new fixture or an adjusted photoperiod, so you can connect cause and effect later rather than guessing. Get AquaPulse on the App Store or Google Play to try it, or see the Water Check feature page for more detail.
Sources and editorial information
- Published
- 8 August 2026
- Last reviewed
- 8 August 2026
- Written and maintained by
- AquaPulse (Valeon Labs Ltd)
This guide draws on specialist aquascaping and planted-tank education sources for PAR ranges, light tiers and photoperiod guidance, since no single official standard exists for ornamental planted-aquarium lighting. Where figures vary between sources, we've presented a range rather than a single definitive number. The algae/light-surplus mechanism and photoperiod starting point are consistent with, and draw on, our own already-published starting a planted aquarium guide. We'll update this page if we find something we've got wrong.
Good to know: This guide provides general informational guidance only. It does not recommend a specific brand or product, and PAR figures are widely used approximations rather than a guaranteed outcome for any specific fixture or tank.
- [1] Aquarium Co-op — "What is PAR? Lighting & Brightness Explained for Planted Tanks" Specialist aquarium-retail hobbyist source, already used elsewhere on this site. Used for PAR as a measurement distinct from watts/lumens and for commonly cited light-tier PAR ranges. Not a scientific authority; figures presented as widely used hobby convention.
- [2] The 2Hr Aquarist — "How to Choose Aquarium Light for Planted Tanks" Specialist planted-tank education source. Used for corroborating light-tier PAR ranges and the light/CO2/nutrient balance mechanism behind algae. Not a scientific authority; presented as widely reported hobby consensus.
- [3] AquaPulse — "Starting a planted aquarium" Our own already-published, separately sourced guide. Used for the 6–8-hour photoperiod starting point rather than re-deriving the same research here.
What we deliberately left out: a specific PAR meter recommendation or brand-specific fixture comparison, since suitable products and pricing change over time; and a precise universal PAR-to-tier cutoff, since specialist sources draw the boundaries between tiers somewhat differently and presenting one exact number as definitive would overstate how settled that boundary is.