Aquarium Lighting Guide

Why Aquarium Lighting Matters

Aquarium lighting does three things: it makes your tank visible and beautiful, it drives plant photosynthesis, and it regulates your fish and plants’ circadian rhythms. Get it right and your plants grow steadily, your fish display vivid colour, and algae stays manageable. Get it wrong and you end up with either dying plants or a tank smothered in green.

The good news is that modern LED technology has made getting lighting right far simpler and cheaper than it used to be. A decent LED light on a timer costs less than most filters and lasts years without bulb replacements.

Types of Aquarium Lights

LED (Light Emitting Diode)

LED is now the overwhelming choice for new aquarium setups, and for good reason. Modern aquarium LEDs are energy-efficient, run cool, last 30,000 to 50,000 hours without replacement, and offer programmable spectrums and intensities that older technology simply couldn’t match. Prices have fallen dramatically — a quality LED capable of growing plants costs less than a comparable fluorescent setup did a decade ago.

For planted tanks, look for LEDs with a full spectrum output (including red and blue wavelengths) and some form of intensity control. For fish-only tanks, almost any white LED strip will do the job.

Fluorescent (T5HO, T8)

Fluorescent tubes were the standard for decades and remain in many established setups. T5HO (high-output) tubes are still used in high-tech planted tanks where even light distribution across a wide footprint matters. The main downsides are heat output, bulb replacement costs, and the fact that output degrades over time even when bulbs still appear to work. If you’re setting up a new tank, there’s no reason to choose fluorescent over LED.

Metal Halide

Once the choice for demanding coral reef tanks and the brightest freshwater setups, metal halide is now largely obsolete for freshwater aquariums. The heat output, electricity consumption, and bulb cost make them impractical when modern LEDs can match or exceed their output at a fraction of the running cost.

Understanding PAR Values

PAR stands for Photosynthetically Active Radiation — the measure of light energy in the wavelengths plants actually use for photosynthesis (400–700nm). It’s a far more useful number than watts or lumens for evaluating whether a light will grow plants. PAR is measured in µmol/m²/s at a specific distance from the light, usually at substrate level.

• Under 15 PAR: Low light — suitable for java fern, anubias, and mosses only
• 15–50 PAR: Low to moderate — most low-tech plants, no CO₂ needed
• 50–100 PAR: Moderate to high — most aquarium plants, CO₂ recommended
• Over 100 PAR: High light — demanding plants, CO₂ injection required

Most beginner planted tanks are best kept in the 20–40 PAR range. Higher PAR without CO₂ to match it causes algae to thrive while plants struggle — the opposite of what you want.

How Long to Run Your Light

For the vast majority of freshwater aquariums — planted or not — 8 hours per day is the correct photoperiod. This mimics the natural day length in most tropical habitats and gives plants enough energy to grow without giving algae the extended period it needs to dominate.

Common mistakes: leaving the light on while you’re home (often 12–16 hours), or running it only when you remember to switch it on. Both cause algae issues. Run 8 hours on a timer, every day, at the same time. Many aquarists set it to run in the afternoon and evening — 12pm to 8pm — so the tank is lit when they’re most likely to enjoy it.

Lighting for Planted Tanks

Planted tanks need full-spectrum light — both the red wavelengths (around 660nm) that drive photosynthesis and the blue wavelengths (around 450nm) that support healthy plant growth and colour. Most quality planted-tank LEDs specify their spectrum; look for ones that cover both ends rather than producing only white light.

For low-tech setups (no CO₂), stay in the 20–40 PAR range at substrate level. Easy plants like java fern, anubias, vallisneria, and hornwort will thrive here. For high-tech setups with CO₂ injection and liquid fertilisers, you can push into 50–100 PAR to support more demanding carpeting plants and stem plants.

Lighting for Fish-Only Tanks

Fish-only tanks are far more flexible. Fish don’t need specific light spectrums — any decent white LED will do. The main goal is providing enough light to see your fish clearly and maintain a natural day/night rhythm. Stick to 8–10 hours per day on a timer. Avoid very bright light in fish-only tanks without plants — it encourages algae growth with nothing to compete against it.

Lighting and Algae

Algae needs three things to grow: light, nutrients, and CO₂. In a tank without abundant fast-growing plants, excess light provides the energy for algae to use the nutrients fish waste provides. The solution is almost always a combination of: shorter photoperiod (8 hours maximum), denser planting (especially fast growers like hornwort and vallisneria), and reduced feeding.

Avoid placing your tank in direct sunlight. Even a few hours of direct sun per day provides far more light than any aquarium light can compensate for, and algae will win every time.

Recommended Lights (2025)

Summary

Choose an LED with a full spectrum for planted tanks, a white LED for fish-only tanks, and always run it on a timer for exactly 8 hours per day. Keep PAR at 20–40 for low-tech planted setups, and don’t go higher without CO₂ injection to match. If algae appears, reduce the photoperiod first before changing anything else. Consistent, moderate lighting on a reliable timer is the foundation of a healthy, algae-free aquarium.