What size aquarium light should I get for my planted tank

There are two basic lighting factors which are essential for a luscious aquatic growth. They are 'quantity' and 'quality'. In lighting terms, quantity is usually expressed in watts or more accurately lumens. The higher that wattage the brighter the light - pretty simple. Light 'quality' is a little more complex since it is based on spectral output. The visible light spectrum is made up of different wavelengths or colours of light as can be seen in a rainbow. For best plant growth, the goal is to mimic sunlight as accurately as possible.

To determine the output 'quality' of a light, manufacturers assign two numbers to a light: colour temperature and colour rendering index (CRI). The colour temperature number is used to indicate the colour of light produced by the bulb, and is based on a physics phenomena known as 'black body radiation'. Picture a hot piece of iron in a blacksmith's shop - as it's heated, it starts to glow red, then orange, then yellow and so on - the hotter it gets the more white it's colour appears. Colour temperature is measured in Kelvin (K), and the colour of sunlight at the equator is about 5200 K, unfortunately colour temperature doesn't completely describe the quality of light, this is where the colour rendering index (CRI) comes in to play.

White light is composed of all colours, with each colour being of equal brightness. The CRI indicates who well this equal balance of all colours is matched by your light source. A CRI of 100 is perfect, any number less than this and the light will still appear white, but it may make objects look bluish or reddish for example. A CRI of 70 or more is probably fine for most aquariums.