Light for viewing
Fish do not need strong light. A basic LED that shows color well and runs on a timer is enough. Extra light mostly grows algae.
Lighting
Aquarium lighting basics
Light does a different job in each kind of tank. Fish-only tanks need it for viewing and a day-night rhythm, planted tanks need it to grow, and reef corals use it as food. In every tank, a steady schedule on a timer matters as much as the fixture.
How to choose
Match the light to what lives in the tank
More light is not automatically better. Buy for the plants or corals you keep or plan to keep, not the biggest fixture that fits.
Light for growth
Easy low-light plants grow under modest light. Carpeting and red plants need stronger light, plus CO2 and fertilizer, or algae takes over.
Shop CO2Light for viewing
Like a freshwater fish-only tank, light is mainly for you and a steady day-night cycle. Reef-strength light only matters once corals are planned.
Light as food
Most reef corals carry tiny algae in their tissue that turn light into food. Reef lights need a coral-friendly spectrum, usually blue-heavy, and enough strength where corals sit.
Saltwater basicsCover the length and depth
A fixture close to the tank's length lights it evenly. Light weakens with depth, so tall tanks need more strength to reach the bottom.
Schedule
Photoperiod, timers, and ramping
How long the light runs each day affects algae as much as how bright it is.
About 6-10 hours
Most tanks do well with roughly 6-10 hours of main light a day. This is a rough guide, not a rule.
- Start a new tank near 6 hours
- Add time slowly as plants or corals settle
- Give fish a real dark period every night
Let a timer decide
A timer keeps the schedule the same every day, even when you are busy or away.
- Plug-in timers handle on and off
- Check the clock after power outages
- Recheck after daylight saving changes
Ease into the day
Many LED fixtures can dim up and down. A slow sunrise and sunset is gentler than lights snapping on in a dark room.
- No ramp? Turn the room lights on first
- Reef lights often ramp blue around a shorter peak
- Keep any night light dim
Light without balance feeds algae
Algae grows when light runs ahead of what plants, corals, and nutrients can use.
- Too many hours is a top cause
- Block direct sun on the tank
- Fix nutrients and maintenance too
Reef lighting
Why PAR matters for corals
You do not need to become a lighting expert, but a few ideas prevent most reef lighting problems.
Light where the coral sits
PAR measures the light corals can use at a given spot. It drops with depth and toward the edges of the light's spread.
- Highest near the top, under the fixture
- Lower in corners and under ledges
- A PAR meter reading takes out the guesswork
Match coral to light
Soft corals and many LPS generally want less light than most SPS. Ask about a coral's needs when you buy it.
- Start new corals lower or to the side
- Move them up gradually if needed
- Move one coral at a time
Acclimate corals slowly
A new or stronger light can bleach corals used to a weaker one. Start well below full power and raise it over several weeks.
- Paling or bleaching can mean too much, too fast
- Watch polyp extension day to day
- Back off if corals pale or stay closed
Keep the light path clear
Water spots, salt creep, and dusty lenses cut the light that reaches the tank.
- Wipe lids and lenses during maintenance
- Fluorescent tubes fade and need replacing
- Acclimate corals after any bulb or fixture swap
Common mistakes
What usually goes wrong
Most lighting problems come from too much, too long, or too fast.
Running 12 hours or more
Long schedules are a common algae trigger. Cut back toward 6-8 hours and put the light on a timer.
Algae troubleshooterDirect sun on the tank
Sunlight adds hours you cannot control and can feed green water. Block it or move the tank.
Green water guideFull power on day one
New reef lights and new planted tanks both do better starting dim and short, then increasing over weeks.
Expecting light to grow plants by itself
Strong light without enough CO2 and nutrients often grows algae instead. Raise light only as plants can use it.
Changing everything at once
Change intensity, schedule, or coral placement one step at a time so you can tell what helped.
Coral not opening