T5 HO Fluorescent Light Guide
How T5 high-output fluorescent fixtures produce broad-spectrum, even coverage for planted and reef tanks, and how they compare to LED and metal halide.
What it is
A T5 HO (high output) fluorescent light is a fixture that uses one or more thin T5 fluorescent tubes. T5 HO tubes are thinner than the older T8 type and provide a brighter, more powerful light. Such fixtures are commonly used over reef aquariums and are also suitable for freshwater planted tanks.
How it works
Several tubes are mounted side by side in a reflector housing. Spreading multiple tubes across the width of the tank produces even illumination with soft, overlapping coverage rather than a single point of light.
Spectrum
Fluorescent fixtures can offer a spectrum well suited to plant growth, and T5 HO tubes are available in a range of color temperatures so a mix can be chosen for planted or reef applications.
Pros and cons
- Pro: multiple tubes give even, broad-spectrum coverage across the tank.
- Pro: more energy efficient and cooler-running than metal halide.
- Con: less energy efficient than LED.
- Con: tubes have a finite life and lose output over time, so they need periodic replacement.
Comparison with LED
LED fixtures are generally the most efficient aquarium lighting and run cooler, while leading LEDs exceed 200 lumens per watt and last around 15,000 to 50,000 hours. Fluorescent lamps last less long and lose output as they age, so T5 tubes must be replaced periodically to maintain brightness. The main advantage of T5 HO over a single LED bar is that several tubes spread light evenly and reduce harsh shadows across a wide tank.
Photoperiod
As with any planted-tank lighting, a daily photoperiod is needed and the light should not be left on continuously, since plants require a dark respiration period at night. New tanks commonly start around 6 to 8 hours a day and increase toward 8 to 12 hours as plants mature; a timer makes the schedule consistent.
Use cases
T5 HO fixtures remain favoured by some planted-tank and reef aquarists for their uniform, shadow-free coverage and long track record, even as LED technology has become dominant.
Sizing
T5 HO fixtures are typically used over medium to large tanks, broadly in the range of roughly 100 to 600 litres, where their wide tube spread covers the full footprint.