Hey Dunc
With an 18" wide tank and 20" high (close to my tank with 8 bulbs), I say you need 8 bulbs for SPS.
6 bulbs won't cover the spread enough, unless you are planning on only LPS.
If you buy an 8 bulb unit, I can help you re-wire it to control each ballast ...
Instead of 2 bulbs 'on' and the other 6 bulbs 'on', we can rewire your fixture as I did mine.
I run 4 ballasts on 4 cords and 4 timers.
As far as dimmable goes, I'm not convinced/swayed about the life of the ballasts and bulbs, so I'm staying away from that option for now.
I've only ever thought about it as I would like to have my lights dim from the last pair in the evening to 'off' when the fish need to go to sleep. But I'm not going to waste my money on a ballast at this time.
As far as your fish and coral are concerned, I don't think they really care.
Think about all the years of Reefers running MH fixtures that are either 'on' or 'off', yet they've had super successful growth.
Save your money and avoid the 'dimmable' feature
Buy an 8 bulb unit and we will re-wire it for you so you can control the ballasts, thus eleviating your concern about running all 8, and you can be cool like me
