Six bulb would be fine but the ati fixture is insanely powerful and the cooling and reflector design allow it to pump out a lot more par than just about any other t5ho fixture on the market, even those that are prettier and more expensive. Don't be fooled by its industrial simple appearance, it performs. At 12" depth you'll need to mount it very high to achieve appropriate par levels. 12-24" high depending on how much light spill you can personally tolerate and what types of corals you're keeping.
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