How to choose the right aquarium light

Updated June 2026Independently researchedNo paid placement.

What size light do I need for my aquarium?

A light’s length should roughly match your tank’s length to avoid dark corners or wasted light. For deeper tanks, you’ll also need sufficient penetration power, measured by PAR at the bottom. A general rule is to look for fixtures that span 80 to 100 percent of the tank’s top length. Many manufacturers offer models in specific inch increments, so measure your tank’s glass length from rim to rim before shopping. For a standard 20-gallon long, a 30-inch fixture may work, while a 75-gallon may need a 48-inch fixture or two smaller units to spread light evenly.

LED vs. fluorescent vs. metal halide: which is best?

LED fixtures have become the clear choice for most hobbyists because they run cooler, use less electricity, and last years longer than fluorescent tubes. They also allow fine control of brightness and spectrum via onboard timers or phone apps. Fluorescent T5 and T8 lights are still used by budget-minded beginners or for very shallow tanks where high PAR isn’t required. Metal halide lights deliver intense output needed for deep reef tanks or demanding corals, but they generate significant heat and burn through bulbs every 12 months. For the vast majority of planted freshwater or mixed reef setups, a quality LED offers the best mix of performance, longevity, and control.

What is PAR and why does it matter?

PAR stands for Photosynthetically Active Radiation, the specific wavelengths of light that plants and corals actually use for photosynthesis. A light may look bright to your eyes but have weak PAR, which means poor growth. PAR numbers (measured in μmol/m²/s) tell you how much usable light reaches different depths in your tank. Low-light plants like anubias or java fern need only around 20 to 30 PAR at the substrate, while a high-tech planted tank or SPS coral reef may require 100 or more. When shopping, look for published PAR maps for the fixture at various heights and mounting distances. Reputable brands provide these so you can match the light to your tank’s depth and desired plant or coral load.

Should I get a light with a built-in timer or controller?

A consistent photoperiod is one of the easiest ways to prevent algae outbreaks and keep your livestock healthy. Most modern LED fixtures come with a basic 24-hour timer, while higher-end models include sunrise/sunset ramping and even cloud simulation. A controller lets you adjust intensity and spectrum without physically touching the light, which can disturb fish. If you’re on a tight budget, a simple mechanical timer paired with a dependable non-programmable LED still works well. However, a programmable controller gives you the flexibility to acclimate new plants or corals gradually and to create a natural daylight cycle that benefits both inhabitants and your viewing enjoyment.

What about color spectrum and Kelvin ratings?

The Kelvin (K) rating describes the light’s color temperature, with 5500K to 6500K being the golden range for freshwater planted tanks, it mimics midday sun and promotes compact, lush growth. Reef tanks often use a mix of cool white and actinic blue LEDs to hit a 10,000K to 20,000K look that encourages coral color and fluorescence. Some full-spectrum freshwater lights also include red and green diodes to enhance the reds of plants and fish, but these are mostly cosmetic. Don’t get hung up on Kelvin alone; a fixture’s PAR output and the actual spectral distribution matter far more for growth. The best approach is to choose a light designed for your aquarium type, planted, reef, or fish-only, and then use its built-in controls to fine-tune the look.

Can I use a freshwater light on a saltwater tank or vice versa?

Using a freshwater light on a saltwater tank, especially a reef, will typically lack the blue spectrum needed for coral health and may have too much red light, which encourages nuisance algae. Most freshwater lights are also not powerful enough to reach the PAR required for corals. Conversely, a reef light over a freshwater planted tank often washes out plant colors with heavy blue light and may be unnecessarily expensive. That said, a few adjustable full-spectrum LEDs can be used for both with careful tuning, check the manufacturer’s suggested settings. For the best results, buy a fixture designed and marketed specifically for your tank’s inhabitants. Mixing them usually leads to lackluster growth or an unnatural appearance.

Frequently asked questions