The short answer
For a first low-tech planted aquarium, choose a fixture that spans the actual mounting points, has a repeatable timer, and can be dimmed. Start with a conservative schedule and output, then adjust from healthy new plant growth and algae response.
Do not buy from tank gallons, wattage, or the phrase “full spectrum” alone. A 20 gallon long and a 20 gallon high have different lengths and depths. The same fixture can deliver very different light at their substrates.
1. Measure the aquarium before comparing output
Measure the outside distance between the points where the fixture legs will sit. Then check:
- the exact model’s adjustable mounting range
- tank depth from light to substrate
- lid, rim, brace, and filter clearances
- whether the fixture is approved for the intended mounting method
Product families often group several lengths and wattages. Fluval’s current Plant 3.0 page, for example, lists separate 22W, 32W, 46W, and 59W models with different mounting ranges. A family name without the exact child does not settle fit.
For common 20 gallon dimensions and the current model decision, use the best light for a 20 gallon planted tank.
2. Match the plant list and CO2 plan
Light demand begins with the plants. Tropica’s current selection system groups plants as Easy, Medium, or Advanced based on light, fertilizer, and CO2 needs. Its CO2 guidance says Medium and Advanced plants need added CO2 within that system.
Use that as a practical sequence:
- Easy plants without injected CO2: prioritize fit, dimming, scheduling, and even coverage. Extra unused peak output is not a benefit by itself.
- A mixed Easy and Medium list: confirm the more demanding plant’s published requirements before increasing light. The CO2 and fertilizer plan may need to change too.
- CO2-injected carpet or color-focused layout: compare light at the intended depth and across the full planting area, then make sure circulation, dosing, trimming, and gas control can support the selected intensity.
A high-output fixture can be dimmed if it has useful control. A weak fixture cannot create more headroom later. The tradeoff is that unused output costs money and makes accidental settings more consequential.
3. Compare the right measurements
PAR is a range, not an intensity number
Photosynthetically active radiation, or PAR, conventionally covers light from 400 to 700 nanometers. University of Missouri Extension explains that PAR names the wavelength range. It does not itself state intensity.
PPFD describes how many photons in that range reach an area each second. For aquarium shopping, a PPFD map or measured value at the intended distance is more informative than an unlabeled “PAR” claim. Depth, water, lid, mounting height, optics, and position across the tank all affect the installed result.
Lumens describe light for human vision
Lumens can help compare visible brightness, but they weight wavelengths according to human vision. Virginia Tech Extension explains why lumens do not always translate well to plant growth.
Wattage is electrical input
Wattage helps estimate power use and distinguish exact models. It does not measure photons at the substrate. LED efficiency, optics, spectrum, fixture length, and mounting distance can make two fixtures with the same wattage perform differently.
Spectrum still matters, but avoid color slogans
Plants use light across the PAR waveband, and different wavelengths can affect growth form. That does not reduce the purchase to “plants need red” or make every blue-heavy fixture unsuitable. Compare the manufacturer’s spectrum information, PPFD distribution, visual color, and control range as a package.
4. Value control before decorative modes
The most useful controls are:
- a schedule that runs without a phone remaining nearby
- intensity control with enough steps for gradual changes
- settings that recover predictably after a power interruption
- separate channels only when you will use them deliberately
Sunrise and sunset ramps can make the transition pleasant to watch, but they do not rescue poor fixture fit or an excessive total light period. Moonlight is optional viewing light, not a reason to illuminate the tank all night.
Fluval lists app control and a programmable 24-hour cycle for the Plant 3.0. If app control matters, also check current device compatibility, account requirements, and whether the saved schedule runs locally after setup.
A conservative startup method
There is no universal photoperiod and intensity for every aquarium. Use a named starting protocol, record it, and change one variable at a time.
For a newly planted aquarium, Tropica’s startup guide recommends six hours of light daily for the first three weeks, then a gradual increase to eight hours. That guidance is designed around Tropica’s startup method. It is a sensible conservative reference, not proof that every established tank must run exactly eight hours.
- Put the fixture on a timer from day one.
- Begin below maximum output unless the exact plant and fixture guidance says otherwise.
- Keep duration and intensity unchanged long enough to inspect new growth.
- If more light appears necessary, change either duration or intensity, not both.
- Record the setting and the date.
- Reverse the last increase if algae expands without a corresponding improvement in healthy plant growth.
Do not react to one old leaf. Nursery-grown leaves may be replaced as plants adapt to submerged growth. Judge the light from multiple species, new growth, shading, and the timing of changes.
What tank response can and cannot tell you
Longer internodes or leaning growth: may indicate insufficient or uneven light, but species form, crowding, and CO2 also matter. Check fixture position and shading before buying more output.
Algae after an increase: supports reviewing the last light change, but does not prove light is the only cause. Check maintenance, plant mass, nutrients, CO2, and recent disturbances.
Pale or damaged leaves: are not a light meter. Verify species requirements, new versus old growth, fertilizer method, planting, and water conditions.
A dark corner: move the fixture or hardscape, trim shading plants, or choose a lower-demand plant for that location. Increasing the whole tank can overlight the center while the corner remains shaded.
A red plant staying green: may reflect light, CO2, nutrition, cultivar, or normal growth form. Do not raise intensity solely to chase a sales photo.
Mounting height and lids
Raising a light can spread output across a wider area while reducing intensity at the plants. Whether that improves the aquarium depends on the fixture’s optics and approved mounting hardware.
A lid, condensation, and surface movement can change delivered light. Compare manufacturer measurements only when their model, distance, and test geometry are stated. A value measured in air at one point should not be treated as a substrate-wide promise in a filled tank.
Use only mounts and clearances allowed by the exact manual. Do not rest a fixture on a lid unless it is designed for that installation.
Electrical and water safety
Follow the light’s manual for mounting, moisture protection, cleaning, and power disconnection. A current Fluval aquarium manual requires a drip loop below the receptacle so water cannot travel along the cord into the outlet.
- Keep plugs, adapters, and controllers out of splash and condensation paths.
- Make a drip loop for every cord.
- Do not use a fixture with a damaged cord, housing, seal, or power supply.
- Disconnect power as the manual directs before moving the fixture or working in the aquarium.
- Use the electrical protection required by the product instructions and local code.
An IP rating applies only to the parts and conditions specified by the manufacturer. It is not permission to submerge the power supply or ignore mounting instructions.
Common buying mistakes
- Buying for nominal gallons without measuring length and depth
- Comparing watts as though they were substrate light
- Trusting a PAR number that lacks distance and position
- Paying for high peak output without useful dimming
- Changing duration and intensity together, then losing the cause of the response
- Running decorative night lighting through the dark period by default
- Ordering the wrong length or wattage from a grouped marketplace listing
- Copying another aquarium’s settings without matching plants, depth, CO2, lid, and mounting height
The decision
For an Easy, no-injection plant list, choose the correctly sized dimmable fixture with the simplest dependable schedule. For a CO2-injected layout, add verified depth and coverage data, then buy only as much output as the full system can use.
Shrimp-centered tanks have an additional viewing and grazing tradeoff covered in the shrimp-tank light guide. If the plant list is not settled yet, choose it first with the aquarium plants guide and the low-tech planted tank guide.