I have the dimmable fixture, but I don't think it's needed, and there are a lot of great coral growers, e.g. Jason Fox, who don't use dimmable and say it isn't needed. If you want to ramp up and down, then definitely get a dimmable fixture. I like it for personal viewing, since it comes on slowly and ends the night with actinics, but I don't think the corals care as much as I do. That's just my opinion and what I have read from some coral growers. Buy what you want though; if you want it to start lower and increase then decrease again at night, go for dimmable.