Coral reefs occur naturally in some of the sunniest places on earth, but light is changed when it travels through seawater. Sunlight can be regarded as full spectrum, but as soon as it hits the water some of that spectrum is filtered out. First to go is the red spectrum, followed by orange, yellow and green. Violet is also filtered out eventually, but blue light penetrates deep and is the last light waves present before the deep oceans descend into darkness.
It’s blue light that corals need the most because they are adapted to photosynthesize in the abundance of blue light where they grow in the oceans. So to replicate that, reef tank lighting should have the right blue spectrum and the right brightness to simulate light exposure on a coral reef in nature.
Use a par meter
The best way to measure light in the aquarium is with a par meter. They measure photosynthetically available radiation and give a number, with zero being dark and 500 being incredibly bright. Low light corals like mushrooms and zoas can tolerate par levels as low as 50, but bright light loving sps corals like Acropora need par levels of 250 or above. Move the par meter around the tank and you see the best positions to place corals, or if you need to upgrade your lighting to provide higher par levels than you currently have.
If you don’t have access to a par meter then most lighting manufacturers will offer advice on the typical coverage for their model, and for which corals.
A four foot by two foot reef tank may only need 2 x 50 watt LED lights for low light corals, but it may need 2 x 90 watts for medium light and mixed reefs, and 2 x 180 watts for bright light tanks like sps dominant. Where you place the lights matters too, as most produce a cone of light so the higher you mount the light, the wider the spread, but the dimmer it will be overall. Place the light nearer to the surface of the water and the higher the par reading will be, but the narrower the beam.
For even lighting, multiple modular LED lights can be used, and if you buy the same brand, they can all be controlled via one app and work in sync with each other.
And if you still need to guess how many watts of high-powered reef-spec lighting you need, here are some pointers:
12x12x12”/30x30x30cm 10-15 watts
24x24x24”/60x60x60cm 90 watts
30”x30”x30”/75x75x75cm 150-200 watts
Note recommended wattage should be adjusted up or down depending on if the reef tank is populated with low light, medium light or high light requirement corals.
Wattage calculations can also be made up of linear LED strips.