How many hours to light an aquarium

How many hours of light per day should an aquarium get? Short answer: 6–8 hours. The long answer depends on what you have in the tank, whether you inject CO₂, and how much light you have. Photoperiod decides whether you have a beautiful planted tank or a war with algae.

This guide explains why 6–8 h is the default, when to go up or down, how to match the photoperiod to stocking, whether a siesta is worth it, and what to check when algae or plants struggle.

1. The default: 6–8 hours per day

6–8 h daily is the sweet spot for most freshwater tanks. It fits:

Community tanks with easy plants and no CO₂.
Fish-only tanks (decorative + observation).
Shrimp tanks.
New-tank startup (first 2–3 months).

Why not longer than 8 h

Plants photosynthesise with saturation — once they hit peak throughput (usually 6–8 h), extra light hours don't accelerate growth. But algae use the extra hours efficiently because they're simpler organisms with higher tolerance to "overdose". Result: longer light → more algae, plants grow the same.

Why not shorter than 5–6 h

Below 5 h:

Plants don't fully "spin up" photosynthesis (takes a few hours to hit full efficiency).
Low oxygen output → fish may breathe faster in the morning.
Plant cycle is disrupted — slower, weaker growth, more disease.

2. Photoperiod per tank type

Fish-only tank

6–8 h daily. Longer light increases algae risk — no plants to compete.
Main role: observation, aesthetics.
You can split into two blocks (2 h morning + 4 h evening) to see the tank at both times of day.

Low-tech planted (no CO₂)

6–8 h daily. Standard for setups with Anubias, Vallisneria, Cryptocoryne, Java fern.
Moderate light (20–30 lumens/L).
If plants grow too slowly — bump to 9 h, but watch for algae.

High-tech (CO₂ + strong light)

8–10 h daily. Stronger light (40–60 lumens/L) + CO₂ lets plants handle a longer photoperiod.
If using EI (Estimative Index) — typically 10 h.
Important: CO₂ kicks in 1 h before lights on, stops 1 h before lights off.

Shrimp tank

6–7 h daily. Shrimp don't enjoy intense light all day — they hide.
Plenty of hiding spots and moss provide natural shade.
Moderate light is enough (20 lumens/L).

Fry tank

8–10 h daily. Young fish grow faster with longer days (mimicking summer).
Dense low planting (hornwort, Vallisneria) so they have cover.

3. Siesta — split photoperiod

A siesta splits the photoperiod into 2 blocks with a break. Example: 4 h morning + 4 h evening, with a 4-hour gap.

What it's supposed to do

A brief halt in photosynthesis rebuilds CO₂ in the water (if you don't inject).
Algae get a "reset" — they don't use the break as well as plants.
You see the tank both morning and evening.

When it helps

Low-tech with algae (especially BBA, staghorn) — siesta sometimes works.
Tanks where you want to see the fish before work and after.

When it doesn't

High-tech with CO₂ — a break desynchronises CO₂ dosing.
Tanks with sensitive fish — constant light changes may stress them.
Planted tanks with light-hungry species — both halves are too short.

Practical takeaway: a siesta is useful as a diagnostic tool (fighting algae in low-tech) or for aesthetics. It's not a miracle or a golden standard.

4. Timer — yes, from day one

The most important photoperiod accessory: an electric timer. Costs $5–15 and eliminates the main source of lighting trouble — "I forgot to turn it off".

Why a timer solves 80% of problems

Consistency: plants respond to a steady rhythm, not your mood.
No more forgetting — one weekend of 16 h of light is enough to breed an algae bloom.
Automatic siesta if you pick a multi-program model.
Holidays — the tank turns lights on and off while you're away.

Timer types

Mechanical (dial) — cheapest (~$5), fine for simple once-a-day schedules.
Digital (LCD) — $10–15, 4–8 programs, supports siesta.
Smart (Wi-Fi, Shelly Plug) — $20–40, app control, automation. Overkill for a single tank but handy if you have several.

5. Light intensity vs duration

Photoperiod is not the only light variable. Just as important is intensity, measured in lumens per litre or PAR (photosynthetically active radiation).

Rough ranges

Low-tech no CO₂: 20–30 lumens/L, 6–8 h.
Moderate planted: 30–40 lumens/L, 7–9 h.
High-tech with CO₂: 40–60 lumens/L, 8–10 h.
Fish-only: 15–25 lumens/L, 6–8 h.

Intensity or duration — which wins?

Intensity × duration = total light dose. Day to day you can reach the same effect through:

Short + intense (6 h × 50 lumens/L).
Long + moderate (10 h × 30 lumens/L).

For beginners: dimming the light is often more effective than shortening the photoperiod if you have an algae problem. Dimming the LED by 20% at the normal photoperiod often beats cutting the day by 2 hours.

6. New tank — gradual startup

A fresh tank is unstable and the first weeks will have algae before plants establish. To minimise that window:

Week 1–2: 4 h per day.
Week 3–4: 5–6 h per day.
Week 5–8: target 7–8 h per day.
Week 9+: optimise for your plants and stocking.

Building up gradually gives plants time to root and colonise substrate before algae get a full light dose.

7. When to change photoperiod

Algae appear

First move — cut photoperiod by 1–2 hours. From 9 h go to 7 h. From 7 h try 6 h. Observe for 2–4 weeks.

If shortening doesn't help, the problem is elsewhere — CO₂, fertilisation, stocking. See the full algae guide.

Plants grow slowly or yellow

Don't increase duration right away. First check:

Fertilisation — maybe lacking nitrogen, potassium, or iron.
Light intensity — lamps may have "tired" (LEDs lose 20–30% of their output after 2 years).
Water hardness — very hard water blocks iron uptake.

Only when those are fine — then bump the photoperiod by 1–2 hours.

Seasonally (holidays, winter)

Some fish and plants react to longer summer days vs shorter winter ones. Not required for a typical tank, but:

Summer: 8 h (May–September)
Winter: 6 h (November–March)

It also matches your observation window — home earlier in summer, later in winter.

8. Natural light — yes or no?

Why not

A tank next to a window (especially south-facing) is a classic beginner mistake:

Light intensity changes throughout the day from 0 to dangerously high.
Direct sun overheats the water (summer 29+ °C).
Unpredictable photoperiod — cloudy day ≠ sunny day.
Excess light feeds algae; plants can't keep up.

When it's fine

Diffuse light from a north or west window for a few hours — acceptable as a "supplement".
Tank several metres from the window, no direct sun.

How to minimise the issue

Curtains on the window or a "black background" on the tank.
Match the LED photoperiod to hours when sun doesn't hit the tank.
Monitor water temperature — if summer > 28 °C, problem.

9. FAQ

Is 12 h a day too long?

For most tanks — yes. Outside high-tech with CO₂, 12 h is a simple recipe for algae. Cut to 8 h and observe for 3 weeks.

Can I run only 4 h?

Short-term (while fighting algae, max 2 weeks) — yes. Long-term — plants weaken, grow slowly, get sick easier. Return to 6–7 h as soon as the problem passes.

Do I need to turn lights off at night?

Yes, absolutely. 24 h of light = a complete reset of the natural rhythm. Fish need night to rest, plants metabolise "nocturnally" (oxygen → CO₂ in respiration), the hormonal cycle needs darkness.

Tank in a bedroom — is a blue "night light" OK?

Better completely dark. A "moonlight" LED (0.5–1 lumen/L) may be tolerated, but it doesn't help the tank — it helps you. If it's a problem, move the tank.

I forgot to turn lights off for a weekend. What happens?

2–3 days of continuous light likely means an algae bloom next week. Return to the normal photoperiod, do a 30% water change, cut fertilisation for a week. Buy a timer.

What colour temperature? 6500K, 10000K, RGB?

For a freshwater tank — 6500–7500K (white with a slight pink accent) is standard. Plants grow under the full white spectrum. "Pink" plant LEDs work, but look unnatural. Adjustable RGB gives the most flexibility.

Do plants need darkness to rest?

Yes. At night they reverse gas exchange (consume oxygen, produce CO₂). That's normal. Without night, plants decline and die within 1–2 weeks.

Does dimming (dimmer) help?

Yes — especially in tanks where the lamp is too strong. Dimming 20–40% gives results comparable to a shorter photoperiod, but plants adapt better. Modern LEDs have dimmers in the remote or app.

Wrap-up

Short crib sheet:

Default: 6–8 h daily. Works in 90% of cases.
High-tech with CO₂: 8–10 h.
New tank: start at 4 h, build up gradually over 2 months.
Algae? Cut by 1–2 h and observe for 3 weeks.
Timer on the outlet — always. A few dollars for eliminating 80% of problems.
Avoid sunlight from a window.

Don't obsess over PAR readings. Don't overcomplicate the siesta. Give plants a consistent, moderate photoperiod and observe — the tank will tell you what it needs in 2–4 weeks.

Related: easy plants for beginners, algae in the aquarium (photoperiod is the first thing to fix), fertiliser calculator (light and fertiliser need to match).