Choose a planted tank light by PAR (photosynthetically active radiation) at the substrate, not watts. Roughly 15 to 30 PAR suits low-light plants, 30 to 50 medium, and over 50 high-light plants that usually need CO2 and fertilizer. Deeper tanks need more output. Run a photoperiod of about 6 to 8 hours, increasing slowly, and balance light with nutrients and CO2 to avoid algae.
Light drives plant growth, and algae growth. Balance is the goal.
PAR targets at the substrate
| Plants | PAR | Usually needs |
|---|---|---|
| Low light (anubias, java fern, cryptocoryne) | About 15 to 30 | Root tabs or light fertilizer |
| Medium light (many stem plants) | About 30 to 50 | Regular fertilizer |
| High light (carpeting plants, red stems) | 50 and up | CO2 and full fertilization |
What to compare
| Spec | Why |
|---|---|
| PAR map by depth | Real output where plants grow |
| Spectrum | Full spectrum with red and blue supports growth and color |
| Length and spread | Even coverage across the tank |
| Dimming and timer | Ramp up and control photoperiod |
| Mounting height | Changes intensity |
Photoperiod
Start around 6 hours and raise gradually toward 8. A midday break is optional. Consistency matters more than length.
Algae control
More light without CO2 and nutrients invites algae. Add fast-growing plants early and change water regularly.
Related: Aquarium Heater Size Guide.
Reef lighting has different needs; see reef parameters. Browse heat, light and flow.
Common questions
How much light do aquarium plants need?
Low-light plants do well around 15 to 30 PAR at the substrate; demanding plants need 50 or more plus CO2.
How long should aquarium lights be on?
About 6 to 8 hours a day is a common starting point for planted tanks.
Why does more light cause algae?
Plants can only use more light if they also get enough CO2 and nutrients. Excess light feeds algae instead.
Where do I find PAR values for a light?
Many makers publish PAR maps by depth. Use those rather than wattage.