Hello,
There is no watts per gallon formula. The measurement is light intensity for the animals you intend to keep. The deeper the tank the more intensity you will need to reach the corals and still provide enough photosynthesis to maintain the colors you desire. The type of ballast and bulbs combinations can have a dramatic effect on PPF values.
NOTE: PPF = Photosynthetic Photon Flux measured in µmol m-2 s-1.
Here's an example I ran across the other day:
I was testing an Odyssea 250W HQI fixture with DE bulbs because the owner was reporting browning of his corals. He had placed the corals well within range (proper distance) from the bulbs. At 15" the PPF was 200 and the unit was just a few weeks old. My first thought was a bad bulb so I put a new Ushio 10,000K bulb in and the PPF was 210. Then I hooked up a Galaxy electronic ballast and put the original bulb back in. The PPF was 550 (these measurements were taken through air). So you can see without Quantum meter it can be difficult to say what type light for you to buy.
I can say with some confidence what PPF is required for different classes of corals though.
50-150 PPF most soft corals.
100-250 PPF most LPS (Large Polyp Stony) corals.
250-400 PPF most SPS (Small Polyp Stony) corals.
550-750 PPF the most light demanding SPS corals.
The above were measured through water.
I realize that the average hobbyist may not have $350 to spend on a Quantum meter so with the above caveat here are some rule of thumb recommendations:
12" deep tank 150W
22" deep tank 250W
22"-36" deep 400W
36"+ deep tank 1000W
Regards,
Kevin
Note: I use a Apogee MQ-200 Quantum meter which has a submersible probe.
