Before changing a light, check tank depth, plant demand, mounting height, photoperiod, coverage, and shaded areas. Most algae problems start with too much usable light for the available carbon and nutrients.
Check the tank before checking the light
Light choice begins with the planted area and the livestock, not with the highest wattage on the product page. A shallow low-tech tank may need a modest fixture. A deep tank with a carpet plant needs more usable light at the substrate. A fish-only tank may need enough light for viewing without creating a long algae-growing window.
| Check | What to decide | Risk if ignored |
|---|---|---|
| Depth | Will useful light reach the substrate? | Weak lower growth or bare patches |
| Plant demand | Low-tech, moderate, or high-tech? | Buying more intensity than the tank can use |
| Coverage | Does the fixture reach the planted footprint? | Hot spots and dark corners |
| Photoperiod | Can the duration be adjusted gradually? | Algae before plants establish |
How should you set spectrum and intensity?
Plants use a range of visible light, but a spectrum label alone does not tell you how much light reaches your plants. PAR measurements are more useful when the measurement depth and distance are known. Treat a manufacturer number as a reference, not as a guarantee for your exact tank.
Start below the fixture’s maximum output and adjust one variable at a time. If new growth is healthy but algae spreads on the glass, reduce duration or intensity before changing several fertilizers at once. If plants stretch toward the surface, check coverage and distance before assuming the spectrum is wrong.
What should you check during installation?
- Mount the fixture securely above the tank and keep electrical connections above the drip line.
- Make sure the full planted footprint receives light.
- Set a timer so the photoperiod stays consistent.
- Leave shaded areas for fish that avoid bright light.
- Clean salt, dust, and water spots from the fixture only as the manufacturer permits.
When should you change the setting?
Change one setting and wait long enough to observe new growth and algae response. A useful lighting plan is stable, repeatable, and matched to the plants you actually keep. Chasing a maximum number is less useful than finding a setting the tank can support every day.
Where to go next
If the checklist says the fixture is fine but plants still stall, the limit is usually carbon rather than light — compare the PAR targets for low, medium and high light plants against what your fixture actually delivers at substrate level. If algae appeared after a lighting change, work through the nutrient and light balance before dimming anything. For fixture choice by tank depth, the lighting guide covers mounting height and coverage.