Planted Tank
Aquarium lighting
Rough wattage and photoperiod by plant demand and tank depth.
Watts-per-gallon is a shopping heuristic for LED fixtures. Prefer manufacturer PAR maps at your substrate depth when available.
Results
Fixture power (est.)
40W
Target intensity
1.00W/gal
PAR ballpark
40–80 µmol at substrate
Photoperiod
7–8 hours
CO₂ optional at this level — consistency matters more than chasing high light. Stem plants and carpets possible with liquid carbon or low CO2. Balance light with fertilizer so plants outcompete algae.
How to use this
- Pick the hardest plants you actually want to grow — not aspirational red carpets if you are still learning.
- Enter volume and water depth. Deeper tanks need more punch to deliver the same substrate PAR.
- Use the wattage estimate to shortlist fixtures, then compare spectrum (full-spectrum / high CRI) and dimming.
- Start the photoperiod short (6h) on a new tank and ramp up over two weeks to reduce algae blooms.
Guidance used
- Low ≈ 0.5 W/gal · Medium ≈ 1.0 · High ≈ 1.5+
- Depth >20" applies a modest wattage bump
- PAR ranges are substrate targets for that demand tier
20 gallon, ~12″ deep — starting fixture watts
Watts-per-gallon is a shopping heuristic, not PAR. A dimmable bar at 50% on a 20 long beats a cheap floodlight at 100%.
| Plant demand | Rough watts | Hours | CO₂ |
|---|---|---|---|
| Low (Anubias, fern, crypts) | ~10 W | 6–8 | No |
| Medium (stems, swords) | ~20 W | 7–8 | Optional |
| High (carpets, red stems) | ~30 W | 8, ramped | Yes, ~30 ppm |
Worth knowing
- A dimmable light beats an oversized fixed light — algae is easier to prevent than to cure.
- Match fertilizer and CO₂ to light; high light without nutrients is an algae farm.
- Mount height and glass lids cut intensity — account for them when reading PAR charts.
Usual mistakes
- Running 10+ hour photoperiods on a brand-new uncycled tank.
- Buying the brightest bar “for future plants” and leaving it at 100% on day one.
- Ignoring hotspots — measure or observe plant health across the whole footprint.
See also: CO₂ · Fertilizer · Substrate