Answer: You have to use a 400w bulb in the 400w ballast -- the wattage has to be the same. Which bulb you prefer would depend on what you are trying to grow, which part(s) of the spectrum are more important for your specific plant(s). There are conversion bulbs available as well as systems that can run either kind of bulb. Your supplier should be able to help you with designing a system to suit your needs.
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